Is there a simple reason for the cross over straps on Marine canteen covers other than Marines just being different? The M1910 had been around a long time before they decided to go the cross strap route.

I have no idea as I do not specialize in gear.  I have wondered this too.  I have one in my collection so am curious as well.  The canteen is WWI era.  It has trench art dated to 1935 on my example. I am a former Marine so my hunch is they just wanted something 'different' but I could be wrong. They had a different helmet in the War on Terror, etc. Good question to post. Here is my canteen example.

When the P41 gear was taking form, the early packs had "dog ears" , too.  I suspect this was a Marine Corps thing and if there was something behind it, might well be lost to history. BTW, the bug on the front of the canteen cover is a nice touch...