Not sure I buy the excuse that the established characters were more popular since they did try introducing several new characters. Tried and failed since if memory serves, most of them didn't even last a full season.
The original intent of the creators was to do a series of season-long arcs featuring mostly new characters. When the show was a breakout hit in its first half-season, the network suits got involved and insisted that pretty much all of the main Season 1 characters had to return. I think that the creators tried to salvage their original course, hence the influx of new characters on top of the established ones. Whether or not the creators' original vision would have worked is questionable, but you can blame a lot of what was wrong with the show with the network interference. It was pretty clear, for example, that they had no idea what to do with Hiro and Ando after Season 1.
They probably could have done a better job of teasing out the show's "mysteries" if each season had featured a mostly-new cast.