That's likely because with their current plans, they only plan to build a mod. There is no direct financial gain for making Blizzard Dota, but as I explained in my previous post, it can lead to an increase of sales for SC2. Seeing as Blizzard Dota does not have a large crew working on it, it is more than plausible this was their intent from the beginning, as there isn't a lot of cash lost for developing Blizzard Dota.