TV shows have a much lower budget and Nathan Lane is a bigger-name actor than Ernie Sabella, plus as mentioned Nathan does a lot of broadway production. He didn't voice Timon in the Timon & Pumbaa TV show from the 90's either, while Ernie reprised Pumbaa.
James Earl Jones only voiced Mufasa in the pilot episode of The Lion Guard ("Return of the Roar"), and they re-use his voice for the title card in each show, but otherwise he has been voiced by Gary Anthony Williams.
Rob Lowe is still the most bizarre choice to me for Simba... I just cannot get used to it (my kids watch the show sort of regularly, but I guess I'm the biggest Lion King fan in our household, lol). It conveys a completely different personality. I really wish they would have gotten Cam Clarke instead -- he has been voicing adult Simba in Broderick's place for many years on several projects and sounds VERY similar to him -- for example Cam voiced Simba in appearances on the Timon & Pumbaa show, he voices Simba in the Circle of Life show at Epcot, and sang for him ("We Are One") in The Lion King 2. I like Lowe in other projects but not this one. He delivers all of his lines very stiffly and doesn't show any of the occasional insecurity of Simba, nor the occasional playfulness. I don't get the choice at all.