JJ45, that is an excellent question and there is a very good answer.
The USMC used the Winchester M70 (Pre-64) until -----------you guessed it, 1964. In 64 Winchester dropped the old action and made the "new push feed M70" and called it the M70, but NO parts on the new rifle will interchange with the real M70. In my opinion the rifle made from 64 until the new controlled feed M-70 came out many years later should never have been called an M-70. It’s not bad at all, but it’s not an M-70 in design. Only in name.
But----the barrels that Winchester were making for the new rifle were not generally as good as what Remington was making, so the M700 was more accurate.
Savage was also very good, but for reasons of production and quite a bit of politics too, Savage was not considered (I wish they had been)
So that left only Remington to provide rifles for the Marines and this all happened in 64-65. What else was happening in 64?
The Vietnam War.
So the USMC needed something and they needed it NOW.
Remington was the only option that they were offered. The DOD and the Navy held the purse strings and made the decisions, not the USMC itself. So the M40 (my old issue rifle) was issued.
But if you look at the current M40-A5 you will see that it is HIGHLY modified.
The boys at Quantico Virginia had to address the extractors; the bolt handles the scope bases, the stocks, the triggers, the bottom metal and even the barrels. In fact the current Marine Corps M-40A5 had ONLY a receiver and bolt body from Remington. Everything else on the rifle is after market.
But when a new weapon is integrated into military systems the weapon is the smallest part of the purchase. The support systems and structures all have to be adapted. So even though Ruger and Savage and the new Winchesters are all WAY better rifles as they come from the factories, the USMC cannot simply just change.
The Marines have the smallest budget of any of the US Armed Forces. The entire support infrastructure has to change if you go to a new weapon and the Marines can't afford that. The Remington 700 has been in the system since 65 and they have to stay with it until there is sufficient money to change. The Armorers at Quantico have integrated improvements in almost every aspect of the weapon in the last 50 years, but the very fact that such improvements were found to be necessary should speak volumes to us all.
All these improvements have addressed failures and weaknesses in the M700 as it comes from Remington. And Remington has not even been gracious enough to sell the USMC actions, but make the Marines buy the whole rifle and then the boys at Quantico strip them and re-build them to USMC current specs.
So when someone says "The Marines use them" as a justification to their belief that the Rem-700 is "the best' it is because they have no knowledge of the facts. We used them then, and my comrades still use them, because we had no other choice. Not because they were "the best". I am not saying they are total failures. But they are absolutely not "the best".
Savage, Winchester and Ruger are all better. So is Howa, CZ, Mossberg, Mauser, Sako. Steyr,and Browning (I do have some reservations about Tikka) As civilians we get what we buy. As a Marine we got what they gave us.
Before Savage went to MIM castings for the extractors you seldom saw one break. In the 60s and 70s they used milled extractors. They are just as accurate as Remington and perhaps a bit better. Their bolt head is floating and they use a collar around the bolt body which is the bolt handle. They NEVER break. Their old trigger was as good as Remingtons and less likely to break or malfunction.
Ruger makes it's bolts with integral handles. They also never break, and in all my years of gunsmithing I have NEVER seen a Ruger bolt fail. The issue triggers are not good enough for use as they come, but that is a small problem to fix. They are as accurate as Remingtons as a rule also.
The "New Pre-64" Winchester is the best today in my opinion. As issued the design need no improvement at all. Put a "Marine proof stock" on it and you have the best "snipers rifle" you can get today in a bolt action. If Winchester were to be asked I'd bet they would even make bottom metal to take 10 round and 20 round detachable mags.
But until the Marines get "wealthier" it's not going to happen.