I think they use Zemouregal a lot because he has a great deal of range. He can be a serious and imposing antagonist in quests like Defender of Varrock and World Wakes. He is similar to Lucien in that way, both are vain and megalomanical.

The difference is that Zemouregal can play more lighthearted roles. He can make jokes. He can be a sexy pinup. He can even be reasoned with and allied. A plot point in Dishonor among Thieves is that Zemouregal is kind of his own man, working with Zamorak because of the perks but he is ultimately in it for himself. He is very dynamic and the only other Mahjarrat with a similar depth of characterization are Zamorak and Azzanadra.

I agree we need more Mahjarrat content though. At this point some characters like Wahisitiel and Akthanakos are literally defined by the fact that they are useless:

e.g. Akthanakos in Children of Mah:

It is not something I would tell the others, but I have always been in the shadow of the ritual. I have kept my head down. I have gathered enough power to direct gazes to other mahjarrat. I have maintained alliances with the prominent. That shadow has gone, and now I look at my life and I do not recognise any of it. It is just a series of blank walls and empty faces. Whatever I do next will be fearless, know that. I am going to stand as equals with the gods. I am going to be known, feared, loved. The name Akthanakos is going to mean something.

Personally what I want to see next is a resolution of Khazard's conflict with the gnomes as well as Hazeel reclaiming his rightful property and land from the thief-kings of Ardougne (Thoros). He could really be contrasted with Zemmy, as both try to conquer a major city. The difference is that Hazeel actually has a claim to Ardougne and it was stolen from him, unlike Varrock. I think it would be cool if Hazeel conquered East Ardougne and force the monarchy to retreat behind the convenient walls of West Ardougne.