The answer, I believe, is that most people don’t realize the damage caused by the indult Mass, or they falsely believe the indult Mass is better than no Mass at all, (i.e., they must have a Mass). Or it is easier to accept the indult Mass to go along to get along. By attending the indult Mass a person implicitly shows that he accepts the new mass because the indult is only allowed under this condition.
Attending an indult Mass gives bad example, as does approval of the indult Mass by the N-SSPX, Bishop Williamson’s group, and other compromise groups.
Archbishop Marcel Lefebvre stated in a letter of Mar. 18, 1989, to Father Daniel Couture, “These Masses are scams which lead the faithful to compromise their principles! Many have already abandoned them.”
If you attend the indult, you get a Faith-destroying sermon. Who can believe a priest trained in Modernist schools and seminary for his whole life would give a Faith-strengthening sermon? Not a chance. This puts your Faith in jeopardy. It’s all a plan to liberalize the faithful to accept the conciliar church. It is offered as a “treat” but is actually a “trick.”
Archbishop Lefebvre understood that the enemies of Christ’s Kingship occupying Rome would not blush at using this Most Sacred Gift of God, the Holy Sacrifice of the Mass, in order to lure the resisting Catholics into the conciliar church. So, his warnings ring true for all the congregations and clergy who compromise (in any way!) with Vatican II, the new mass, the new Code of Canon Law, the new Profession of Faith and other conciliar changes! Archbishop Lefebvre initiated a Catholic Resistance; we simply must continue resisting modernist Rome, “until we have a perfectly Catholic successor of Peter.” (Letter of Archbishop Lefebvre to Four Bishops of SSPX, Aug. 20, 1987.)
To improve relations with Rome, the N-SSPX publicly thanked Rome for a (false) “freeing” of the Mass, (the July 2007 motu proprio), which can be used only by a priest who accepts the new mass. Because of this and many other N-SSPX compromises, including unnecessary sacramental indults (for confession and marriage), uncompromising traditional Catholics should not attend N-SSPX Masses or support the N-SSPX.
By attending the indult Mass wherever it is offered, or by supporting the groups who approve attending it (e.g., the N-SSPX and Bishop Williamson’s group), you put your Faith in jeopardy.
Further, former Pope Benedict XVI wrote:
The faithful who ask for the celebration of the forma extraordinaria [i.e., the Latin Tridentine Mass] must not in any way support or belong to groups which show themselves to be against the validity or legitimacy of the Holy Mass [i.e., the conciliar mass] or the Sacraments celebrated in the forma ordinaria.
I believe attendance at the indult Mass would be down if the priest saying the indult would start out by explaining each time that “If you are attending this Latin Mass under the Motu Proprio, you acknowledge that you accept the new mass. If not, please leave and don’t come back.”
Not only did the N-SSPX thank Rome for the (false) "freeing" the Mass, they explicitly accept 95% of VC II. Yet, many still overlook this, believing God will understand. He is certainly merciful, but He is also just.
Below are other reasons for staying far away from the indult. Would you attend the indult Mass offered by a priest who also offers a Black Mass? If not, then why accept a priest who offers the sacrilegious new Mass? The new mass is inherently harmful to souls and to the Catholic Faith. Thus, it is a sacrilege because it fits the definition of sacrilege: viz., the “irreverent treatment of the sacred.”
If a person attends an indult Mass in a church that is also used for the new mass, then he willingly uses for worship a place which is also set aside for the commission of sacrilege. A place used for sacrilege is far more unfitting for Divine worship than a “neutral” place, like a rented meeting hall.
Thus, when traditional Catholics compromise by choosing an indult Mass, they accept the conciliar hierarchy’s bait and they presume they will be strong enough and wise enough to discern and reject the accompanying conciliar poison.
That is the exact excuse friends of ours made to us in the late 1960s, when we left our local diocese parish in order to remain traditional Catholics. They felt we should stay to fight on the inside and help the parish reject liberalism. However, absolutely none who stayed on the inside remained faithful to the Catholic Faith. None.
The question arises as to whether or not the indult Mass gives grace. Although we know that God gives grace to whomever He pleases, whenever He pleases, He does not give grace because we offend Him (as we would offend Him by attending an indult Mass).
In conclusion, Archbishop Lefebvre called the indult Mass a scam, i.e., “a fraudulent or deceptive act or operation; to deceive and defraud.” He also stated in his Aug. 29, 1987, letter to the four new bishops of the SSPX that Rome permitted the indult Mass “in order to lure the resisting Catholics into the conciliar church!”
So, let’s heed the faith-saving advice of Archbishop Lefebvre and stay completely away from all indult Masses and all compromise groups accepting indult Masses, like the N-SSPX.
 The Vatican has clearly said that it continues on the course of post-conciliar errors and that these motu proprio (indult) Masses are designed to integrate traditional Catholics into the conciliar church. See, e.g., this article, where the Vatican calls the traditional Mass an “ecumenical bridge” and outlines the plan for using it as a step to blending the new mass and the traditional Mass. http://www.zenit.org/article-32593?l=english – see also, http://www.catholicnews.com/data/stories/cns/1101922.htm.
Cardinal Mayer (former president of the Ecclesia Dei Commission in charge of indult groups) candidly admitted the conciliar trap: “Sometimes a temporary solution may be necessary, such as allowing them [viz., conservative Catholics] the possibility of celebrating the Holy Mass [of Pope St. Pius V].” See, Letter of Cardinal Mayer to Msgr. May, L’Homme Nouveau, March 19, 1989 (emphasis added), quoted at: http://www.sspx.org/motu_proprio/attendance_at_the_indult_vanes.htm
 May 13, 2011 Instruction “Universae Ecclesiae” on the implementation of the Motu Proprio “Summorum Pontificum”, ¶19 [bracketed words added for clarity].
 Summa, IIa IIae, Q. 99, a.1.
 Quoted from the Merriam-Webster Dictionary https://www.merriam-webster.com/dictionary/scam