The Rov of my Shul spoke about this last Shabbos.
He explained that in medieval Europe trade guilds controlled various trades.
Many small towns and some larger towns would only allow non-Jewish bakers to operate commercial bakeries (Jews were barred from these bakery guilds). This is the reason use of Pas Palter was so widespread and became normative.
However, Jews were revolted at using breads that had keys put inside of them at Easter time so they wouldn’t buy the breads that week. The guilds then relented and allowed the Jews to bring their own keys to be put into the breads they buy.
This was a way to get around the problem and it eventually became a Jewish custom.
It also was a way that Jews identified bread that was baked after Pesach since a key is unique and is a sort of Simman.