There is a problem with themes - themeless masters, that have neither keywords nor big enough pool of models to support their theme.
For example what will be theme for Seamus? Belles? All two kinds of them? But there is usually 1-2 of them on start, because they are summonable anyway, same problem with Nicodem and McMourning, no keywords, no clear theme, no models to support it. I'm sure, that there are similar masters in other factions.
At the same time there are "super-themes", like M&SU, there are at least four M&SU masters and lot of models with that keyword, so it'll be easier to stick to theme and still have broad range of models to choose from.