There is actually some reason behind this, and it’s mostly to do with two factors.
Frist one, is that large capacity magazines are often unwieldy, cumbersome, expensive to make, and unreliable, the same problems as regular magazines at the time just way worse. Also, magazines were often very finely machined, with way sticter production tolerances than the rifles themselves, and even then each magazine was often fitted to individual guns and the interchangeability of magazines was not a guarantee between the same pattern of guns. What’s more, it’s harder to shoot prone or from a trench parapet with a long box sticking out from under the gun.
Best example of the issues of large capacity magazines in service is the Swiss Army Scmidt-Rubin rifle, who during peace time adopted a 12 round magazine to replace the 6-round one, but quickly realized that this actually made the rifle allround less practical and easy to shoot, and with all previous reasons too they decided to remove them from service before WW1 even began.
They tried it, didn’t like it, and went back to their original 6-round magazines.
Second reason, the double-stack double-feed design of the James Paris Lee 10-round magazine was unparalleded until WW2 in terms of making a practical, compact, reliable and non-too expensive detatchable box magazine for a service rifle. The man is often credited (for good reason) for inventing the detatchable box magazine in the first place (or, at least, the guy who made the first practical design) in the late 1870s, this magazine contained only 5-round and he was slowly refining it over the decades, leading to a 8-round single stack Lee-Metford magazine, which then was widened to allow the 10 magazine we’re familiar in WW1 and 2 contexts.
Until… What, the SVT on the eve of WW2, practical detatchable box magazine for rifles were just not a reality outside of the Lee design, and by the time technology had improved to allow alternate designs to begin competing, the world was already moving on to semi-auto rifles. There was, quite frankly, no need to revolutionize bolt-action rifles, when it made much more sense to just go over to the next newest thing in the civilization tech tree.
Detatchable magazines for bolt-action rifles also are not as big a advantage as some people make them out to be…
If they were, other nations would have scrambed to adopt rifles similar to the Lee-Enfield (or, at least, adapt their own designs to allow magazines), but apart from notable exceptions that prove the rule (the Swiss, for example), that doesn’t appear to have been the case.
And, the British wouldn’t have been trying to replace their Mk III rifles with a .270 cartridge Mauser style rifle before WW1 either, if it was so damn effective. This is what the P14 was meant to be, before WW1 forced the Brits to abandon the new cartridge idea entierly, and set up production of the half-way finished replacement design prematurely inside of the US.