Why don't developers join to Trisquel development?

Trisquel is actually downstream of a project that includes non-free software. Ultimately helping fix distributions which include non-free software helps just as much if not more than helping just Trisquel. You can't really help fix bugs in non-free programs because the sources aren't available. So when you submit a bug to Ubuntu which includes non-free software chances are you are submitting a bug for a free software application really. If your not Ubuntu can't fix it either.

But if you back up many bugs are actually upstream of Ubuntu and similar distributions. These distributions get the code from elsewhere and its there that many of these bugs are actually getting fixed.

If you submit a bug fix for Trisquel chances are it'll end up going to Ubuntu and then upstream to the component in which needs to be fixed. IE the mainline kernel project for instance. And really there are even a lot of in-the-middle projects between Trisquel and the actual developers. Trisquel is based on Ubuntu which is based on Debian which is based on code from different components from places like the mainline kernel project. If the bug is mainline kernel submitting a fix to Trisquel, Debian, or Ubuntu doesn't really make any sense. And in this case there would actually be another project too. Linux-libre which sort of forks the mainline kernel (it's not technically a fork... but for all intensive purposes it sort of is in our explanation). And that means that unlike with Debian/Ubuntu which take the mainline kernel as is Trisquel takes a derived version of the mainline kernel called linux-libre.

It makes the most sense to test as far upstream as possible and submit bugs there rather than here unless the bug is specific to something downstream (ie a non-free piece of software is included in Trisquel would be a downstream bug most likely). Although not always. Sometimes it might be linux-libre kernel project that would be best to file a bug report against-or both in some cases.