Why aren't test planes remote controlled?
Taking a plane that is designed to be operated by humans and retrofitting a system for remote control would be a big task. They do it with old fighter jets for use as target drones. But a target drone's task is pretty simple; fly like a fighter jet until it gets shot down.

At the point where SpaceShipTwo is, a bunch of testing has already been done. This is supposed to show that the vehicle is safe enough to move on to piloted testing. But testing aircraft and spacecraft is a dangerous job and things happen.

Creating a remote control system for an spacecraft like SpaceShipTwo would be much more of an issue. First of all, there will most likely be physical changes required, whether this is to install hardware to manipulate controls, or just electronics to remotely operate the flight computers. These physical changes will be extra work and will affect the tests, since when the plane is actually operating, those things won't be in place. And if those remote control systems fail, you lose the aircraft. Sure, you don't lose pilots, but why lose a plane just because a remote control system (which won't be used on production) fails? Should you spend time and money developing a complicated remote control system, or developing the actual vehicle?

Also, many tests require the pilots to be on board. They are not just testing the systems, but how the pilots interface with them. Having real pilots actually flying the vehicle is the whole point of this phase of testing.

See this related question as well: Why do the engineers need to be on board during testing?