Because he wants to expand them beyond that one joke and add emotional depth, and because it'd be rather difficult to get people to take a dramady seriously where the characters look as they did in the original?
What got me was Fuller saying this: "And you have Herman, who is essentially a zombie, living in a constant state of decay, married to a woman who doesn't age." And put so bluntly—I mean, the show didn't explore that at all.
He also mentions at points that he feels like toning down the look and upping the emotional honesty is gonna make it much easier to accept some of the goofier things hes planning for the show.
That, and he wanted different takes on the characters so that it wouldn't feel like he was trying to replace or outdo the old guard.