Why are there more vertical takeoff than horizontal for spacecrafts? Vertical takeoff requires disposable rockets (would it a satellite rocket), which is a money loss, and also a lot of fuel, because initial velocity is zero. Also vertical takeoff seems risky, involves huge pieces of equipments, launch pads, to diminish risk.
Horizontal takeoff are done with a reusable aircraft, like a modified 747. Initial velocity not being zero, there are much less risk, and the fuel spent in a 747 is much less expensive than a disposable rocket.
So, Why are there more vertical takeoff than horizontal for spacecrafts?
 A: A 747 - can get you to around 35,000 feet. Still very much within the atmosphere.
So what do you do then? Launching a rocket from that point still requires an awful lot of kit, so while you have reduced your propellant requirements a little, the 747 still has to carry a launch platform, so you're not really getting anything out of this.
New technologies, such as that used by Virgin Galactic is managing to make this work, hopefully, with a hybrid model that does fly up to around 50,000 feet before launching the spacecraft section, but this is very new.

So the simple answer is - it used to require vertical rocket launches, and all the associated paraphernalia, but modern technology is moving towards fully reusable methods such as this.
A: A 747 moves at approximately 1,000 km/h, a satellite in orbit travels at 28,000 km/h. So, after your rocket is released from the 747 it still needs enough fuel to accelerate a further 27,000km/h. That requires a lot more fuel than the 747 is capable of carrying. Remember that the shuttle lift-off weight is about 2,000 tons - far more than the 747 can carry.
In addition, you need to get above the atmosphere. Going 28,000 km/h at 10,000 m will burn up any spacecraft, so you need to get up very high before you start going at extreme speeds.
It is possible to reach orbit after horizontal take-off, but we're not there yet
A: Any system that has enough ISP to get itself out of the atmosphere has enough ISP to not need wings for lift. Thus, vertical takeoff. See this related question for a discussion of why the ISP of jet engines is so much lower than for rocket engines.
Note also that just a few weeks ago NusSTAR was launched from a Pegasus rocket on a modified 747. This is a relatively lightweight cargo and thus was fit for what the 747 could carry.
A: Short answer: payload!
To launch a reasonable payload you ill need a colossal airplane (and maintenace, runway, operation, etc)
There are the pegasus system using it and you can see he got a very limited payload
http://en.wikipedia.org/wiki/Pegasus_%28rocket%29
A: Sabre engine-powered skylon aircrafts are the only horizontal takeoff spacecraft in the horizon that will be able to go into LEO and back in a single stage.
I expect that we will see a prototype of this aircraft doing its first test flights in the next 10 years, but there are many financial and political roadblocks that need to be surmounted first
