Well in this one you have to take other things into account, why does everyone prefer Modular Powersuits better than let's say "obsidian armor" mods?
Quality and features.
More like MPS being completely broken, you mean. Nothing like having infinite free flight, quantum-level runspeed and a 7-in-1 multitool for the cost of a some bars of iron and a few diamonds. It may not be
your reason for using the mod, but for the large majority it is. "Gravity Suit? Aint nobody got time fo dat", they say, in pursuit of instant gratification over progression-based unlocks. Why build the expensive thing if another mod does it cheaper?
You can't compare GregTech and Forestry for this reason. Forestry in its default config, aptly titled "EASY", is really ridiculously easy. It basically barfs free energy at you, and in contrast to the beliefs of those people who never tried them out, the multifarms are in fact very high yield for very minor fertilizer costs. Except oh no, the upfront cost of the blocks and the requirement to spend 10 minutes mining for apatite, ain't nobody got time fo dat.
Both Forestry and MPS have harder difficulty modes, in addition to MPS being far from finished - it just got updated with the long-announced heat mechanic in 1.5.x, and people are throwing a fuss over their cheap godmode armor actually coming with a downside now. Hardly anyone ever uses these modes though (I do, but I don't know anyone else who does). GregTech, on the other hand, requires investing time into a progressive tech tree, and that makes things "difficult", quote unquote, and therefore unpopular. Ain't nobody got time fo dat.
So no, it's not about quality or features. It almost never is. A few special mods like Thaumcraft can get away with requiring effort because they do it in such a unique and cool way that it's hitherto unseen in the mod community, and others like IC2 for example make themselves essential by providing the framework upon which an entire ecosystem is built. For the rest, it's about giving the player more power, more resources and more energy for less time and effort, less upfront cost and with less complexity. That's what makes mods popular.