The date of Easter varies each year because it is linked to the cycle of the Moon, relative to the cycle of the Sun. In order to set the date of Easter one must first look for the vernal (spring) equinox, which is March 20. The word “equinox” refers to that time when the length of day and night are equal. It is also the date we set for the official beginning of spring. 
 
Having set our sights on March 20, we next look for the first full moon following March 20. Some years, the first full moon occurs quickly, within days of the equinox. Other years it occurs weeks later. 
 
For the Jewish people, this first full moon after the equinox also signaled Passover. And since it was at the Passover feast that our Lord Jesus suffered and died and rose, we Christians always fix Easter to coincide with Passover. 
 
So then, Easter, (which is always on a Sunday since Christ rose on the first day of the week), is celebrated on the Sunday following the full moon after the vernal equinox. 
 
Historically there were great debates within the Church in the East and the West about setting the date of Easter. The system described above was finally settled upon. But today, we still find that the exact date for Easter varies a bit in the western and eastern parts of the Church since many of the Eastern rites still use the more ancient Julian calendar, rather than the Gregorian calendar used by the church in the West. 
 
Your wish for a fixed Day for Easter, as is the case with Christmas and other feasts, is understandable. But as you can see, the relationship of the Moon relative to the sun doesn't fit perfectly into our modern systems of timekeeping and to fix the date as you suggest would probably open old debates that caused great harm in the early Church.

Your desire to pray this prayer is understandable, and good. The prayer can in fact be said. However, perhaps a little history and context is appropriate to understand why it fell away in the early 1970s. 
 
Historically, the liturgical movement beginning in the 1940s and continuing through the Second Vatican Council., sought to reemphasize the Eucharistic liturgy, by distinguishing it from some of the devotions that had grown up around it. The hope was to emphasize participation in the Mass as the greatest devotion. 
 
Among the devotions that attached themselves were a number of prayers said following the dismissal from Mass. Thus, although the priest turned and said Ite Missa est (Go the mass is ended), this was not exactly so. First there was a blessing, then a recitation of the last Gospel, and then after most masses, prayers, which included the prayer to Saint Michael. In many parishes Benediction of the Blessed Sacrament was also done. 
 
Liturgists of the time sensed that lesser devotions following the greatest devotion somehow implied an inadequacy in the prayer of holy Mass. 
 
Whether or not you agree with all these points, it was the thinking at the time, which led to the elimination of many, if not all, devotions immediately following the Mass. 
 
That said, you and fellow parishioners, are not forbidden from praying certain prayers and devotions following mass, even with the priest. It is best however, to allow those who need to depart, e.g. for work, to leave prior to the beginning of devotions. Otherwise, people feel trapped, and the instruction that they may go, is lost or reduced in meaning. 
 
Please also be aware, while the St. Michael Prayer is an important prayer, many others also insist on other devotions for similar reasons. Thus some pastors are reticent in fostering such public devotions, since requests tend to multiply. So pastoral discretion is needed, and solutions will vary from parish to parish.