It's a very strange thing that I've been asking myself as well. From how I understand it, it's the same laser (blue) in HD-DVD and Blu-Ray, contrary to popular belief that HD-DVD is a "red" laser system.

However, I think the control chips for the laser is hardcoded to read the data from a specific format - it passes a stream of data from the disc. It's this part that would require "upgrading" and I don't believe it's programmable.

Combo drives can exist, and have been able to for the best part of a year now:

http://www.news.com/NEC-unveils-chip-to-bridge-Blu-rayHD-DVD-divide/2100-1041_3-6124799.html

It seems cost is the only real barrier due to royalties - that and the major manufacturers have pledged allegiance to one camp or the other and wouldn't be seen dead supporting the other one.

Basically it all comes down to politics, as nearly everything eventually does.