Seasons 1-3 were produced by SyFy, who I assume still owns the rights to that content. Amazon picked up the rights and wrapped it up (one of two good things Prime Video has done so far in my opinion).
Massive budget cuts (compare the last couple of seasons with the first few) and then they skipped the end of the story without even finishing the last season.
The good news is that the books are excellent and I highly suggest them if you haven't had the chance to read them.