So have always wondered why the pre 64 winchester model 70 ( ruger 77 as well ) have two piece floor plates instead of just one solid unit?

Money. It is much, much easier to manufacture a 2-piece trigger guard & floor plate assembly than to machine Mauser style bottom metal with the trigger guard and integral magazine box made from solid block of steel. The Winchester system cost less to make & made it easier to sell the M70 at an affordable price. Same logic applies to the very utilitarian (cheap) one-piece, stamped bottom metal on Remington M721 & M722s.

Yes, and even more money with you consider that all post WW2 rifles use the stamped magazine box to form the feed lips, and the receivers are not caliber specific for the most part. Additionally, two or three lengths of actions, meant that it was not economically feasible to make such pieces of hardware.
Consider that even during WW2, Germany, and the US, started using stamped and welded sheet metal bottom metal on their bolt action rifles.
It just costs too much for zero performance gain and the rifle market would not sustain that, and still won't.
Even the famed Model 70 was doomed to fail as a viable commercial salable commodity; milling the receiver from a solid block of steel was something from the dark ages; Winchester was losing money on every rifle they sold before 1964. The rest is history.