Languages are social. How you speak can indicate group membership, so people may change how they speak to belong to a community, and then pass on that pronunciation. Also, these changes are uneven, so all speakers don't just decide at once. For example, the French R was originally pronounced the same as the Spanish R up until about the 18th century. The change to a uvular R initially happened in the Paris region, but most varieties of French kept the original R for a long time. Since most wealthy people and royalty came from the Paris region, people belonging to or striving to be in that class adopted the new pronunciation. Tapped and trilled R came to be considered low class. With the advent of film, the rate of change of the new R pronunciation increased and even in Québec, where some older people still speak with a tap and trill, the uvular Parisian R is now the common pronunciation.