There are a few reasons why FMJ bullets are used by 95% of combat troops. First they comply with the convetion with no gray areas to debate, second (and most important) they are the cheapest and easiest bullet to mass produce, and third, they are less likely to get deformed during combat which could cause a gun to jam.
Machine guns and battle rifles don't need the accuracy of BTHP match bullets so there's no point in wasting the money to supply them to anyone but snipers. Also, the fewer variations you have in the supply system, the less likely things are to screw up.
On a side note, the only enemy we have fought against who had signed the conventions was WW2 Germany.