78. The Order of Readings sometimes leaves it to the celebrant to choose between alternative texts or to choose one from the several listed together for the same reading. The option seldom exists on Sundays, solemnities, or feasts, in order not to obscure the character proper to the particular liturgical season or needlessly interrupt the semicontinuous reading of some biblical book. On the other hand, the option is given readily in celebrations of the Saints, in ritual Masses, Masses for various needs and occasions, votive Masses, and Masses for the dead.
The “seldom” option extends to the ordinary time celebrations of the parish patronal feast, and to the parish dedication anniversary, but not to Sundays of major seasons. This is also the reason why wedding Masses are not permitted on certain solemnities and feasts. But there is no prohibition, for example, on getting married on many of these days, just on having the readings and prayers from the votive Mass for marriage.
These options, together with those indicated in the General Instruction of the Roman Missal and the Ordo cantus Missae,  have a pastoral purpose. In arranging the liturgy of the word, then, the priest should “consider the general spiritual good of the congregation rather than his personal outlook. He should be mindful that the choice of texts is to be made in harmony with the ministers and others who have a role in the celebration and should listen to the opinions of the faithful in what concerns them more directly.” 
Notes 103 and 104 above are from the old GIRM. It’s a principle we’ve run across many times: options and adaptations are invoked for the good of the people, not the ministers.