Because this a.m. and p.m. thing it's not used in all countries. For example in Portugal instead of 3 p.m. we say "3 da tarde" which is "3 in the afternoon" or use the 24h format.
That and lazyness in wanting to know something (the p.m. and a.m. thing for example), which in the end is irrelevant, it can make it confusing, and because of lack of interest in learning it.