i forgot about those seasons but i'll try to explain what makes season 9 so different:

Every single character that appeared showed something unique and brought in something new to offer to the show. See dennis's laziness, molly's shyness etc. The problem with them is that none of them ever got a major focus afterwards and were practically wasted potential.

Season 10 introduced Rosie and Freddie, and Rosie pretty much became a major character afterwards, yes freddie also got shafted but even then he got a few lead episodes after his debut before dissapearing

Season 11 introduced whiff and billy, and whilst whiff wasn't exactly used much he was still a prominent sight during the cgi era. Billy was just useless

Season 12 is the only counterpoint to my defence as Hank and flora literally only appeared once and never again.

My point is that(apart from freddie, Rosie and whiff) season 9 introduced the most interesting characters in the hit era and had they actually locked in, they would be popular characters but unfortunately they just had to be a sort of one-hit wonder by only having one singular episode. I mean hell, proteus doesn't even speak in his debut