It's a little facetious to say that Walt and Jesse were dropped. They were never intended to be lead characters. The show is about Saul Goodman (hence the Better Call Saul). Of course, they could make cameos, as Vince Gilligan, the Breaking Bad creator, said:
“Personally, I’d have a hard time resisting putting all these guys in
for a cameo or two every now and then.”
However, the show is focussed on Saul. The actor who portrays him, Bob Odenkirk, commented:
“Here’s what I told Vince and Peter – Pete Gould is also writing that
show –we’re gonna make it fresh. It’s not gonna be Breaking Bad 2.0 or
whatever. It’s a different show and it’s gonna have a different energy
and we’re not gonna try to just extend Breaking Bad. That can’t be
done. That show is great, it’s done, it’s all wrapped up and
delivered, and now we’re gonna do something fresh and new with a
character that is rich and funny, and everyone’s excited about it. And
we wanna make sure the audience sees right away that it’s not a
continuation of Breaking Bad, but rather its own thing that hopefully
is gonna be entertaining on its own terms.”
So to summarise - the show is simply not about them. It's about Saul. They may or may not make cameos (both have expressed interest), but they are not the focus of the show.