I think it happened because Lucas never planned for a follow-on TV series when he was creating the prequels. That came after. It's possible he looked back at the sequels and saw how much he overlooked. Though I think the primary purpose of Ahsoka was to provide a viewpoint on Anakin, and give him someone to talk to about Jedi stuff that he wouldn't say to Obi-Wan.
I would have added a Padawan friend and another master instead. In addition I would add a planet that is a center of conflict regularly, so we get to know it and where consequences could carry.
Maybe a republic blunder (like deciding its people are not valuable enough to protect despite the desires of Palpatine, or what he says are his desires) gets him angry at the Republic and the idea of democracy in a more pronounced way.
I'm with you, Shrieking Banshee. I found the Clone Wars dull, with predictable action and poor characterization. And it does nothing to make Anakin's slide into evil any more believable.
For me, the core problem was that they built on top of the prequels which were even worse. Being stuck with continuity, the Clone Wars were stuck. The prequel Jedi had two huge problems: forbidding any attachment - even love of one's mother; and their continuing use of a slave army.
Both of these were supposed to be seen as minor flaws in the heroic good guys that lead to their defeat. To my view, the prequel Jedi are horrible. They are a cult who recruit by pulling children at a young age from their mothers. They indoctrinate them as child soldiers, and teach them that the right thing is to feel nothing for their family. They then accept an army of even more abused children. Those aren't flawed good guys - they are only even vaguely palatable because the Sith are over-the-top evil.
The Clone Wars had an opportunity to walk back some of that - but instead it doubled down on it for the most part. If I were doing something like this, I might have introduced the idea that the Jedi Council on Coruscant had turned to ways that were more efficient, but weren't part of the original Jedi tradition. Maybe an older rural Jedi who is opposed to these things.