"Once up to speed, as much as 600 mph, the vehicle would then switch to rocket engines. separate from the maglev carrier, and propel itself into space. This would dramatically reduce the cost of getting to space because the carrier and electrical power used for maglev are inexpensive and stay on the ground, unlike rocket fuel which adds weight and cost to a launch vehicle."
http://liftoff.msfc.nasa.gov/News/2000/News-MagLev.asp

The reason why not is because it is expensive, but if you wanted a launch site on Hawaii and the govt wanted spend billions of dollars, it might be plausible.
But you could sell it as an airport as much as a spaceport. A commerical jet airliner uses a lot fuel just in it's take off- so it could instead be accelerated by mag lev, it could also be used in landing to get electrical energy by regenerative braking. You could build on eastern side of either the mountains on Hawaii.
You could sell on idea that for "international" or air traffic coming from the US mainland you would lessen air traffic pollution, both chemical and sound pollution.
Now, mountains are generally not on a steep angle- most roads people drive on don't exceed 12%- to avoid steeper grade roads can wind up a mountain. Part of the "air port" could also provide a road for cars that goes straight up the mountain and straight down the mountain- the cars could also be Mag Leved- thereby giving a perk to the locals.
But the angle of this track is unlikely to exceed a 45% angle- though if you had track at 45% it exceed the angle that most airliner takeoff at. And of course rockets normally launch at 90 degree angle.

Now suppose this mag lev track charged $1 per ton going up with rebate of $.10 per ton from energy going down. Thereby costing locals $.75 per ton for local to take shortcut to other side of mountain.
For airplanes: "The gross weight of the 747-200B is 836 000 pounds, more than that of almost any other aircraft ever built."
http://www.globalsecurity.org/military/systems/aircraft/b747.htm
So at 836 000 pounds a airplane would pay $418 to take off- plus typical landing fees and got $41.8 for rebate for landing if same weight. Now, you might charge more for airplanes and not give rebate, but it would probably cost more than $500 in jet fuel to take-off from normal airport.

"At brake release, we will accelerate to about 150 mile per hour in less than 30 seconds and liftoff consuming about 5,000 pounds of fuel an hour, per engine. At cruise altitude, the fuel usage rate for each engine will decrease to about 2,000 lbs per hour. This increase in fuel efficiency is why jetliners fly so high.

Like the missile burning fuel, we will consume 1,500 pounds of fuel as we climb just getting to 20,000 feet. By level-off at 36,000 feet, we will have used two tons of jet fuel at about $2 a gallon ($1,200 total). Nearly two and a half hours downrange, our center main tank will be empty and we will run the rest of the way to our destination on our full wing tanks. Over the planned 5:05 hours of flight time to Phoenix, we will consume 25,500 pounds, or over eleven tons of fuel ($7,700!). Even at that rate, I have read that the fuel used is still less than if we had loaded 40 or so SUV’s with three or four people each and driven everyone of our 137 passengers the same distance."
http://www.blogsouthwest.com/2007/02/12/we-have-liftoff/

So this isn't a big plane [137 passengers] and the pilot says cost $1200 of fuel to reach 36,000'. If you accelerated this plane to 12,000' and to speed of 400 mph, could probably reduce the fuel cost to reach 36,000' by around $1000. And you might be able to design this airport so that if accelerated dead weight [a block of concete] to 400 mph the trajectory of it falling would go beyond to towns and population centers thereby in sense removing these areas from most of the risk from accidents.

Now with say an Atlas IIIA rocket it weighs 472,534 lb so the charge for launching at 400-500 mph if at $1 per ton would be $236. Again you might charge much more, like $5-10,000 per launch, but if it only cost $10,000 to launch that would a huge cost savings.

Now, you could build this first for local traffic, which expands to air traffic and then rockets. Now if you only had couple hundred cars traveling "over the hill" per day this would amount to far more mass per day than what would possible in at today's rate space travel- though air traffic could easily exceed car traffic. And in terms of decades into the future it's conceivable that space travel traffic could exceed air traffic- especially if you consider sub-orbital travel space traffic.