The Little Mermaid TV series is a prequel, the time frame being about a year before the movie. ^_^

At any rate, I don't know why Disney didn't make a TLK tv series featuring the lions, but I can't say I'm not thankful for that. Perhaps they felt that it would cheapen/detract from the films or that fans would have serious issues with possible character butchering?
Timon and Pumbaa are basically comic relief, so they're more versatile - it was probably easier to adapt them to a children's television show where stupid jokes and bathroom humor reign supreme.