When I ask vendors why, they generally tell me that they don’t have the in-house resources to do the testing. In other words, they are cheap and don’t want to spend the money.
While I completely agree that this is true in many cases playing devil's advocate I know that there are some smaller houses who really don't have the time, money or resources to keep up with the latest new sql releases all the time.
Take someone who's just purchased sql server 08 last year and spent a year developing their new software product and need to get it to market and get sales for the next year or 2 to recoup their investment and pay the bills let alone have any extra for product upgrade, not to mention support costs of the existing product.
Situations like this come down to their prices and sales volumes and business feasibility and unfortunately there are some software vendors who struggle day to day to make more than a small margin and just manage to stay in the black.
So I'm not sure if "generally" is the right word but then again maybe I know more smaller vendors than larger ones so my perspective is more skewed in that regards. :-D