Logic-level transistor not switching
If the lamp is really 0.75A @ 5V, then it's resistance is closer to 7 ohms (when hot). 

The Vgs of 2.5V is the threshold, where it will just start to conduct, so you have not chosen a good MOSFET. However, it probably would work, if the load resistance was a constant 7 ohms. But, it is not a constant 7 ohms, a cold filament can have 10 times less resistance than a hot filament. So, you never get enough current to heat up the filament.