I'll take a crack at a cliffs notes version
In the 1960s, the church called the second Vatican council. Among other things, revisions to the mass were planned, mainly in the document Sacrosanctum Concilium. This document called for some pretty organic changes to the Mass. Use of the vernacular (not exclusively), increased participation from the laity, adding an increased focus on scripture, etc.
In the decades that followed, the more progressive (from a liturgical standpoint) wing of the church took advantage of the situation and implemented changes that I'd argue were considerably more aggressive than the council intended.
Instead of introducing the vernacular in some spots and maintaining Latin for others, Latin basically became non-existent in many masses.
Instead of new and appropriate music traditions being incorporated, with chant maintaining its "pride of place", music became a free for all, with folk and other modern musical forms becoming the norm.
What used to be a very defined structure to the mass became a menu of options to the priest to choose from.
And all of these things aren't necessarily bad, but they certainly can and have been abused.
But I think the biggest loss is that a lot of the traditional calendar and Catholic culture got thrown away at the same time. There are a few things that the Council never got around to discussing that disappeared. And a rich treasury of prayers that were used in the old (pre-Vatican II missal) was chopped up, rearranged and partially discarded. A lot of "hard teachings" were de-emphasized.
The Latin Mass (often called the TLM, Traditional Latin Mass) preserves all those things, quite simply because it uses the old missal. I was first attracted by the aesthetics of it, but as I learn more, I'm lessed concerned with the loss of the "Latin" and moreso with the "Traditional" part.
As for why it is being banned/restricted: From 1970ish-2008, the TLM really existed in a bizarre state. It required special permissions to say, and was not very widespread. In 2008, Pope Benedict changed that with a document that essentially said the two forms could coexist.
However, a portion of people who love the TLM also reject some other church teachings, like the entirety of the second Vatican council, whether or not the Pope is the pope, etc. Francis feels that this view is widespread, and that the two forms cannot coexist.
tldr; Pope Francis feels the Latin mass is associated with a conservative/traditional (occasionally to the point of schism) wing of the church and must be eliminated.
