probably because the next version could run completely differently so it might not be compatible? I'm not a huge nerd when it comes to these kinds of things but it doesn't seem very unlikely to me that the database will not carry over, if that makes sense.
Viewing post in Tentacles Thrive Beta v0.1 (NSFW) comments
But Nono has said this:
**1. Web save/load uses browser cookies, which can get cleared. It is not hugely reliable, as well as not likely to be compatible with future web versions, unlike the downloadable files.
So if you downloaded it like me, we should be good. Though I am not entirely sure where the save is stored in the first place, to be honest.