There's a big player in economies of scale. If you have spare parts for an A-29 on board whatever, you have less room for parts for an Osprey. It's one of (many) reasons Cobras and Hueys go together. They have significant parts commonality.

There's also (probably, without looking) zero parts left for an OV-10 or A-29 in the military supply system, so rebuilding inventory and/or manufacturing capability is difficult, at best. The supply chain for the Osprey is "mature" now, we know what breaks at what interval, and it's easier to support them now.

I don't know what burn rate is for an Osprey, but yes, I am certain that cost per hour is higher than just about any fixed wing, though most of that is probably from maintenance.