Because, contrary to popular belief, the Church had a vested interest in spreading its message to people, and that meant speaking to them in their own language at the very least. Sure Latin would work for anyone who was someone, the clergy ofc, a large number of nobles (though not all), and increasingly the urbanized and wealthy "middle class" of wealthy merchants, landlords, and other non-noble figures who were making large amounts of money later in the Middle Ages, but for the uneducated masses, the language of their birth would likely be all they were familiar with (and maybe a smattering of other languages if they needed them in their line of work/life) and its doubtful that they had any formal literacy, certainly not in Latin, and therefore a great deal of the Church's work was not available to them through the language barrier right?
Well it is not as if the Church was unaware of this as a problem! The Church then, as a matter of practicality, had to include vernacular languages as a part of its ritual, or at least adjacent to it, and by the 1300's the evidence for vernacular sermons is clear, and that is only in the official confines of churches, vernacular speeches by religious figures were far more common, going back to the 11th century in England at least there were prominent figures, such as Archbishop Wulfstan, who delivered speeches in the vernacular languages with at least a somewhat broad audience.
Throw in with this mix a new breed of wandering preachers and religious figures in the Franciscans and Dominicans, starting in the 13th century, along with a greater public presence of the Church as an institution. That is the short answer to your question really, the Church moved increasingly towards the inclusion of vernacular languages as well as the increased presence and visibility of figures such as friars and Domincans, literally the order of Preachers, who brought Christianity, or to be more specific a more orthodox form of Christianity, to the masses, and you have the ingredients for the explosion of popular Christianity in Medieval Europe.
Now we can break this down even further though, because not all religious instruction took place inside a church.
We can then look at a variety of elements that contributed to the spread of Christianity among the people of Europe (who were already Christian to be clear, but might not be on the same page as Rome on any number of issues). Consequently no discussion of medieval Christianity is complete without reference to the visual medium of instruction, and I don't mean lectures on a chalk board (not that they used chalkboards at the time but that's another issue)
When I refer to the visual medium here, I mean the literal sights of Christianity (accompanied by sounds and smells to be sure) that helped instruct and inform the people.
Sights like this
St. Chappele, the royal chapel established an patronized by St. Louis (Louis IX of France)
All of these visual media, architecture, paintings, manuscripts, were used in the visual language of the Church to highlight teachings. Elements such as the centrality of Christ's human suffering, the importance of Marian devotion, the grandeur and spectacle of churches and chapels that called up to heaven, all of these were methods that the medieval Church used to inform viewers of the message that was also conveyed in homilies, sermons, and the preaching of the new mendicant orders.
So it was a confluence of a variety of factors, newly visible Church efforts through the new Holy Orders that were established in the 13th century, the growth in the visual media of the time and the development of new artistic styles that caught on all helped bring the orthodox Christian message to the people of Europe.
