In the days of the old Corps, Gen. Van Orden (ret.) used to sell modified M70s as snipers and worked to get them adopted by the Corp. The first non-military rifles used for sniping purposes by the Marines was indeed the Win. M70. They were later replaced by the Remington 700.
While the M70 can do the job, there is a whole line of developed products or proven techniques already engineered for the Remington. Also, the earlier crop of current M70s had some quality control problems with regards to the receiver and the recoil shield. We have an old thread (now locked) which discusses this.