The first 5 series of the show are legitimately some of the best educational Schumer I've ever seen on television, regardless of country or region. An amazing cast of talented actors, brilliant musical sequences, gorgeous sets and costumes, and writing that can be enjoyed by both adults and kids. to not watch the first five series of Horrible Histories is to do yourself a massive disservice. The first five series are, collectively, fantastic.
That said, I'm not sure exactly what happened, but the 6th series is a complete and total mess. They must have been operating with a quarter of the budget of any of the other series, as it's just a lacking experience in every possible sense of the word. The practical sets are minimal and mostly replaced with crappy greenscreen backgrounds, the writing is completely derivative and lacks any of the charm or brilliance of the rest of the show, and they use puerile fart jokes and poop humor to fill in many lulls in the dialogue during sketches. Also missing are the genuinely awesome songs and phenomenal cast. It's a complete mess.
It's my understanding that this was produced somewhat as an afterthought, several years after the end of the proper show, which explains a lot about its lack of quality. I'm glad they ended it here, rather than stretch this garbage out over another few series.