It could be because there's a 15k character limit on Tapas, which is around 2500 words. Either way, definitely wouldn't recommend writing directly onto any site. Glitches happen, power goes out, devices fail before you can save. Like everyone else said, maybe try Google Docs and make sure you back up your work somewhere else, even if it's just e-mailing yourself the files.
Sorry this happened to you and best of luck with your writing!