Because:

(1) Rockets are proven technology, nobody has yet been able to put anything into LEO using a railgun

(2) Rockets have significant freedom when it comes to choosing the final orbit to deploy to

(3) Rockets are relatively cheap on the 'capex' front, a 200 km long electromagnetic launcher up the side of a mountain will be extremely expensive and may not even work...

Of course if you're so sure about this then maybe start a company to create an electromagnetic launcher, it should be easy to raise money assuming you are right.