Apparently, they already had the games running on other systems, but licensing problems with some third-party software component in the game prevented them from releasing it. As far as I know, the license holder for that component is unclear, since the original license holder does not exist anymore.
I hope that Sega sees this as an opportunity to iron out these issues and get the games out there.
Here is the post I was referring to.
It seems there are legal/copyright problems prevented HD ports from happening:
[Source: http://www.shenmuedojo.net/forum/viewtopic.php?f=5&t=47323]