In no particular order:
1. Cos they ain't Tau.
2. Cos they ain't Eldar.
3. Cos they ain't a Necron Decurion.
4. Cos its stupid to spend the money and do nothing with them.
5. Because Ork vehicles are awesome.
6. Cos they ain't Eldar.
Don't forget, orks are green, not grey plastic coloured.