Possibly because they are proceeding very deliberately through a test protocol that involves advancing the engines a little bit more on every flight.
They want to make sure that once they put the engines in the full-forward position, they can definitely bring them back into the vertical position.
Typically flight testing on a prototype is done very, very carefully. This goes double for aircraft of a new type, or that are using a new technology. In this case, there is new technology in a flight-critical system, namely the rotor tilt mechanism.
Also, they don't want a repeat of the painful development process of the V-22, during which there were several fatalities (followed by many more fatalities once the system had been accepted by the military).
In the current age of drone advanced remote control technology, why do they need to risk the lives of test pilots at all?