I actually think that Flower child quit the forge because he didn't like that his mod was compatible with other mods.
Biggest reason there... Initially, I think Flowerchild believed that compatibility with other mods meant that he could share and collaborate with others with his work, and by doing so expand his mod. However, the ultimate reason why he began hating this was due to the fact that the compatibility altered and 'deformed' the game design of his base mod by other mods and their modifications. Specifically, when I began showing my work on Youtube with the BTW community, they became flabbergasted at the level of change I could institute with simple CR designs and tech tree alterations. Alternatively, I ended up with some of the most unique styled progression paths in the game with my recipe 'addons', and I do think this is why FC began to leer away from compatibility as a whole (because when FC began to make some rigid, semi-unreasonable changes to the BTW base, I was usually the first to straight up circumvent said changes, lol...)
So basically, the Forge leaving is a move to basically "preserve the mod" in his mind. Which, tbh, is pretty damn vain overall due to how modding works period. FC can try and 'justify' his move as much as he wants, but really it boils down to him wanting his mod 'played his way'...
That's a tall order to ask from the very same people who won't play MC 'the way it was intended'...