1. The clear data in GS2 (and also in 1, I think) let you do a new game +. That's all it does in GS2. If they did intend for it to carry on, they would certainly have made the password option available as well, since they would assume the next game would be on GBA as well.
2. The developers did not plan another GS. They made GS 1 and 2 a single story, meant as a standalone pair. Why keep the data if you don't plan on using it?
3. GSDS features the descendants of the earlier protagonists. This means that it's highly likely the original cast is either dead or old. I highly doubt you'll get to use Isaac and company. But the save data savedtheir stats, items and djinn. What good does that data do for the newcomers?
That's three good reasons that you can't import clear data. Of course, that doesn't mean you won't be able unlock anything for having the clear data, you just don't inherit all the stuff from the first two.