This was covered in various threads here and at the BIS forums if I'm not mistaken. Basicly, the various mod's who looked into it were either busy with other projects, or had other issues.
One major problem is that with the Stryker, there are a whole pile of different manned and remote turret options. Remote turrets are reported difficult to implement in OFP, (I think mainly for MP), and the the number one issue of course is the hard-code in the engine preventing rocket and wheeled vehicle combinations.
That said, I still would like to see one built. The btr-70 addon out there would give some good ideas for prototyping a Stryker.