You said “only a few” but I showed to you that there are 19 combos, and many of those combos have more than one build guide. It is still more than 52% of the total number of possible combos, that is not the definition of “only a few” 
As I write in that post I linked to, very seldom patch changes KILLS a beginner build guide. Some item might get changed, might get a bit weaker or stronger but it can still do the job it is supposed to do.
That is why there is more than one build guide for each kind (well channeler caster is only one but I think Nery is working on fire or aether AAR now). I also include many build ideas (without guides) that might be a suitable budget farmer build.
It is most likely a combination of both. I can only speak for myself here.
- Some class combinations and skills are easier to level with and build around than others for beginner farmer builds (e.g. what MI’s have strong item skill modifiers, faction items, etc)
- Some class combinations do not have a variation of viable late end-game playstyles (e.g. Saboteur, there is only one “real” end-game build for that, the Nex and Ortus dual wield build) this will lead the player into a narrow hall with a dead end. This is not good IMO, I prefer to make guides for class combos that have more than one viable late end-game option.
- It is also a matter of time vs. interest. You can read pretty much anywhere on the internet that most people suggests Warlord, Cabalist, Oppressor, Sentinel, Warder, Vindicator etc as first starter class combo. Hence it is natural to focus on guides for those class combos (as I mentioned in a previous point) since they will be read and used the most.