Again, this happens a lot in show business: older/veteran performers are quietly shoved off in favor of newer, younger, fresher blood, and it almost seems as if SST is no exception: Chris, Leela, Mando, and now Nina have all been added to the show in recent years, and they're younger - Nina almost seems as if she's an entire character replacement for Maria.
There's a lot of reasons why this kind of thing happens, but in cases for kids shows like this, a lot of it is because studios and such feel that kids will better relate to younger characters than they would older characters, hence why we see a lot more of the aforementioned younger characters and a lot less of older characters such as Gordon or especially Susan and Bob. Back in the days when we did pledge drives at PBS, our last couple of pledge drives slowly phased out our long-standing on-air personalities and brought in local teens and kids for on-air duties because they wanted to show kids watching that kids are actually involved in supporting PBS (it was one of many attempts they made to get try to increase their incoming pledges since we were losing more and more pledges each year).