Commonality is a big thing, especially in wartime. If I have equipped all my soldiers with the M1 Garand, which is a .30-06, I'm not going to buy additional M1s in the newer .308 without a good reason. That would just needlessly complicate things. This influences what calibers that commercially available rifles are chambered in because manufacturers aren't going to go through the hassle of clambering in rarer calibers just because.

The transition to the .308/7.62mm was ultimately political. NATO members wanted it so that everyone was on the same standard. This then trickled down -- the majority of rifles were available mainly in NATO calibers, especially after the move to 5.56mm. Nowadays these two calibers are kind of the default, though you can find specialty exceptions like sniper rifles chambered in .338 Lapua.

This rationale also extends to bigger guns. 105mm was the most popular postwar big gun caliber until 155mm came along. We also sometimes see weird parallel development. The famous German 88mm (8.8cm) would see its shell repurposed and used in the gun on the Tiger (with different primer). Some calibers would essentially just be rebranded: the famous .50 is often referred to as 12.7mm despite not changing and not actually being 12.7mm.

So how did the original standard wind up being the original standard? Usually because a military commission, like the one the Germans cobbled together after the advent of smokeless powder, evaluated different calibers and then picked one. Once that commission decided in the 8mm Mauser, that became the standard. Deviation was frowned upon. Germany adopted that caliber alongside the Gewehr 1888 and would keep it until the postwar era. It would only develop a competing cartridge in WW2, when it needed something heavier than the 9mm Parabellum and lighter than the 8mm Mauser for early assault rifles.

Hope that gives you a general idea on how calibers were selected.