Well...a few reasons...
One is (now this is the old, original M77, not sure about the Mk II ?) how the Front Bedding Screw pulls down on the action.
Unlike most bolt actions that pull straight down, vertically, the Ruger M77 action pulls down and forward, at an angle.
For pulling down onto an angled wooden stock this makes a certain amount of sense.
But from a strict accuracy/bedding standpoint this angle allows the action to slide, not being pulled down tightly onto a solid bedded platform.
That's (a big) one.
Another is that the shape of the tubular shaped action of the M700 allows for more rigidity and also allows other ways of supporting it steadily (like "V"-Blocks).
The tubular design also lends itself to easy machine work in a lathe for truing the face of the action.
This second feature is not really anything against the Ruger (or Winchester, or Mauser etc.), just a "plus" for the Remington design.
Another plus for the Remington is their (seperate) trigger, which is pretty good (and tuneable) as delivered, and the fact that many after market makers make special triggers for the M700.