Former carrier pilot here, so I'm familiar with the output of the old steam systems (having used/lived-and-not-died by them).
You would think that steam is better, and in terms of simplicity of energy supply, it might be: Run a bunch of pipes maintaining the steam energy levels, and hit the button--boom! You're done.
However, reality is hella more complicated. Old catapults were one-shot; you loaded up the steam, and hoped to hell that the spike in acceleration didn't break your aircraft (thus leading to a lot of over engineering of the aircraft and very careful quadruple-checking that you have it set for the right weight/speed/etc.)
Newer catapults were progressive--you add steam along the travel of the ram, and the acceleration was smoother. However, that means that you have to have multiple valves that function exactly right--enough of them go wrong at once, and you're just going to launch a $30+ million aircraft and crew into a minor speedbump in front of the carrier.
The ram itself is very impressive. Carriers have a couple hanging on the walls of the hangar deck, and they are monstrous--I don't have any stats, but they are like 30+ foot long torpedoes that have to be accelerated with the aircraft, then stopped in a very short distance. When carriers launch, you feel the entire ship shake from this massive metal rod hitting the front end of its track.
Then there's maintenance. We're talking live steam here, not the piddling crap that comes from your tea kettle. It's "dry", as in superheated and has to be kept that way. But that means complicated insulation, piping, and constant checking that you're not eating away at your metals in this environment. Not only to you have to keep it at the correct condition before using it, but you have to do something with the used steam--which means an equally complicated recovery system.
All of this adds up to a massive effort to slingshot some dumbass (speaking as one) off the front end of a ship so he can use equally complicated gear to try to stop him after a cycle or two.
Steam works, but only because it was the only medium that could do the job at the time. I don't know the details of the EM rails, but I'm sure that the final design probably uses electric/electronic analogs to the system...but you can replace a bad circuit board or switch a helluva lot easier than you can a bad valve or piping. That, and more refined control of the overall launch makes this an obvious evolution.