The main issue with concepts like rail guns and what-not for orbital launches is atmosphere. Your atmospheric drag is a function of velocity squared times the density of the atmosphere- a very non-linear function of how close to the ground you are.

Rockets have a huge advantage in that they go slow near the ground, where the atmosphere is thick. They're not dealing with as much air resistance. As they reach altitudes with less atmosphere, they can throttle up. This saves a lot of fuel, mass, energy.

A rail gun, however, can only add velocity, energy, at the beginning, and will slow down every moment for the rest of the launch. This is the worst possible condition. You're at your highest velocity while you're in the thickest part of the atmosphere. You'd need to put an incredible amount of extra energy into the system because of that.

So maybe you put the end of your rail gun on the top of a very high mountain. But the construction costs, the transportation costs, all of this, probably makes it simpler to just use rockets instead.

See also:

https://en.wikipedia.org/wiki/Project_HARP

https://en.wikipedia.org/wiki/Project_Babylon

[Edit] Also the German V-3. An important note is that the V3 and Babylon (unsure of HARP) were designed to have multiple charges go off as the projectile proceeded down the barrel, which gives an acceleration profile more like a railgun than a traditional cannon.

https://en.wikipedia.org/wiki/V-3_cannon

Solid-fuel rocket boosters were used instead of explosive charges because of their greater suitability and ease of use. These were arranged in symmetrical pairs along the length of the barrel, angled to project their thrust against the base of the projectile as it passed. This layout spawned the German codename Tausendfüßler ("millipede").