Why are Marines (US) not "soldiers"?

They are under different regulatory bodies:

They are indeed, but that’s not the reason why surgeons aren’t called Dr. The Royal College of Physicians also only represents hospital drs, not GPs.

Which is further supported by the fact that there is no Marine Corps service academy. Graduates of the Navel Academy have the choice of becoming commissioned officers in the Navy or the Marine Corps. In fact about 1/3 annually choose to become Marine Corps officers.

Because of convention, although it’s not absolutely followed by everyone.

As an example, the National WWII Museum seems to follow the convention capitalizing “Marine” and not “Soldier.”

About the museum:

The museum is famous among WWII buffs, holding conferences, publishing articles

and hosting guest speakers whose lectures are posted on YouTube.

While they are not the final arbiters of style, their style reflects the usage by historians when discussing WWII.

Random example of “Marine”

Note the capitalization of “Marines” and none for “troops” despite the fact that the troops were US forces.

Random example of “soldiers”

Note that both “soldiers” and "troops are lowercase, again despite the fact that the troops were US forces. Also note that “forces” are is also not capitalized.

If this were a discussion among linguists, I’m in the descriptivist camp. Prescriptivists can argue whatever they would like, but this is not what is actually used.

Also note that the convention of chapitalizing “Marines” in not universally adhered to, but I do not recall “soldiers” being capitalized.

My WAG is that “Marine” and “Marines” when used to describe the troops are usually capitalized because they are closely related to “the Marines”, the institution, where “soldier” and “soldiers” is a completely different word than “the Army”, meaning the US Army.

And that marks the end of my participation on this side discussion.

As was noted upthread, the New York Times style guide and the Associated Press style guide both dictate the capitalization of “Marine” but not “soldier.” This is likely why historians typically follow the same pattern, as you point out.

However, the various branches of the military are the ones dictating that “Soldier,” “Sailor,” “Airman,” etc. be capitalized in their official communications. I was simply pointing out that if you are going to capitalize “Marine,” why not capitalize all service members for consistency and like their service branches apparently prefer? It seems a little odd that only “Marine” would be singled out for capitalization by convention.

Finally, I’m not aware of anyone advocating for the capitalization of “troops” or “forces”.

Do you believe that the style requests from all organizations be followed, or just ones from the US military?

As said earlier, I’m following the same style as history books, as I’m used to it.

Early 1990s, I’m at an indoor soccer match to watch my cousin, a new Marine, play. Just a few minutes into the game he breaks his nose and we drive to some military medical facility for him to get treated. Naturally, we are having to wait.

I, about 12 years of age, had heard my cousin disparagingly refer to the Navy as “squids” in the past, so I made some comment about how we were having to wait because of “all these squids”.

I guess that’s a real insult, because I remember him telling my to be quiet, considering that Navy doctors were the ones treating him. He seemed pretty serious.

It isn’t a real insult, no more than jarhead. They’re ribbing terms and that is all.

Now “Self-propelled sandbag” might not go over as well with some Marines. OK to use it among themselves, but us Squids and Snipes need to be more careful about using it, never mind Army or civvy.

Yeah, it’s just some mild ribbing and all in good fun.

That being said, Moriarty does explain that his cousin was a new Marine and you really shouldn’t be using them if you’re some fresh-out-of-boot nub.

IME, squid is a bit derogatory while jarhead not as much. People have tried using jarhead in a derogatory manner but it isn’t so bad. Now if one said dumb jarhead our stupid jarhead, obviously that’s different.

In the broken nose story relayed by @Moriarty , the guy was there for some care and medical treatment so you want to avoid using squid in that scenario. You’re at their mercy, and how you’re treated can be more painful if they’re not careful. Yeah, don’t piss them off. Basic common sense.