You pretty much have all the answers.
Semiautomatic was not accepted at first for a number of reasons, I'll list them in order from greatest factor to least.
1: COST. Way back when they had a habit of producing MILLIONS of military rifles. Just in case. The semi automatic design was tossed out because it increased manufacturing time and cost by far too much to be considered economical. Contrary to popular belief, semiautomatic designs are more expensive and complicated than fully automatic design. Not to mention all the time soldiers would need to learn how to operate them.
If a weapon was semiautomatic it was assumed soldiers would shoot more rounds than necessary, thus adding to the cost.
2: RELIABILITY. Semiautomatics of the time were extraordinarily new and VERY complicated. They knew that war was getting dirty and unsophisticated so a reliable gun was a better gun. Early semi-autos were far from this.
3: ACCURACY. The locking systems in semiautomatics are not as strong as those found in bolt action rifles. The pressure of the round had (and still has) the tendency to escape through the ejection port and many leaks in the bolt itself. Not to mention gas was needed to push the piston back loading another round. Less pressure means less velocity. Less velocity means less accuracy (In the vast majority of cases.) You very well could hit a target out to 800-1000 yards with a bolt action rifle of the time, but barely hit a 300 yard target with a new semiauto.
4: STUBBORN WAYS. Many generals didn't feel the need for them. Many generals had fought in previous wars with nothing but either single fire rifles or muzzle loaders, so bolt action was already a major step up for them. Semiautomatic was just ridiculous. This however did not happen to select fire intermediate caliber rifles (Assault Rifles) Nearly as much. The only notable major leader to deny the assault rifle was Adolf Hitler.
And to be fairly honest, I would have much preferred a Mauser or Enfield back in those days to many semiautomatics we have now. Back then trenches were hundreds and hundreds of yards apart at times, and semiautomatics were not, and are still not capable of such ranges. I am speaking of intermediate calibers of course, I'm fully aware that larger calibers beyon .375 are capable of those ranges, but their weight and odd caliber variation turn me off.
Anyways, I hoped that answered your question. Feel free to ask more, I love answering