With a few notable exceptions like the German 88, I've noticed most gun calibers have been the same (37, 57, 75, 90, 105, 155, etc.) even when used by varied nations. I was curious as to why totally different nations arrived at using the same diameter shells as their rivals/enemies.
One, interchangeable parts. It's incredibly advantageous to be able to share ammunition between two allied nations.The other thing is probably diminishing returns and efficiency. As a round gets larger the amount of powder needed to propel it gets exponentially larger on top of that portability goes down. So the name of the game becomes finding the optimal effectiveness to resource expenditure. Given that it becomes fairly easy to see why certain numbers are settled on by multiple nations.
The simple explanation is that countries did not independently arrive at these calibers, it's caused by lots of different militaries around the world purchasing their artillery or leasing the designs for their artillery from a few lead manufacturers (mostly from Europe). During the late 19th/early 20th lots of new designs were requested by different countries. The manufacturers would come out with different sized guns and if a gun was made in quantity that size could be become a de facto standard. New requests would be for the same size. The German Army, for example, would over the years order multiple different "105 mm" guns from Krupp or Rheinmetal. Other countries would then buy the same gun (or lease the design) and then they would request new "105 mm" guns. Most of the calibers you see today date back to this time.
It's important to note the the ammunition wasn't always reusable just because the guns have the same diameter.
So why so few companies? By the beginning of the 20th century what we would see as a modern artillery (pre-tensioned steel, rifled barrels, breach loading, modern recoil systems, etc) had been invented. But designing these guns and the ammunition to go with them had become very expensive in terms of money and technical expertise. So the number of companies capable of actually brining out a new design shrank over time.
Even relativly rich countries like the U.S. were dependent on foreign designs. For example, during WWI the standard heavy field gun for the U.S. Army was the "Canon de 155 C modèle 1917 Schneider", a design from the French manufacturer Schneider. The U.S. Army used french-build guns and a U.S. built version of the same gun, the "155 mm Howitzer Carriage, Model of 1917 (Schneider)". This was eventually replaced by the "155 mm Howitzer M1" in 1942. Note the gun diameter remained the same and even today countries all around the world use 155mm guns.