Well Gaim was popular, simple as that. It got a lot of anime fans to pay attention to Rider. Judging from toku twitter in the last day, there's still a lot of people very excited to see more of the characters.
The Toei Tokusatsu Fan Club streaming site is currently uploading Gaim in batches. They clearly believe doing so will attract Japanese fans to subscribe. There's also a new CSM version of the Sengoku Driver, which is being released in part because when Team Rider USA polled American fans to see which Driver they'd like to get a chance to buy, it was one of the most popular ones.
So yeah, product to sell, money to make, new specials to promote it all. It makes sense, even though I agree that there are other Rider shows I'd like to see get specials, or even other characters from Gaim that I'd have rather seen return.