I think the biggest reason why we only got a Martial Power 2 book when we did was because Backgrounds hadn't been designed at the time the first one was released. So barring just an article in Dragon, making MP2 was the best way to create and distribute Martial Backgrounds to people.
In addition, the Martial classes were also the ones that seemed to generate the most talk about "missing" or "wanted" builds, which I'm sure prodded the designers to design them (the same way the desire for a dual-wield fighter build was the hue and cry prior to the first MP). A ranged warlord build, a rogue build built around stealth, a thrown-weapon ranger build... these were just a few of the desired builds that have been talked about for a while.
I actually have heard much less talk or complaint about "missing" divine, arcane and primal builds... which gives you a pretty good idea that the designers were more in the zone and had more experience designing for 4E when they got to the first sets of those books. Most complaints with those power sources are geared toward the class mechanics that they wish would just get "fixed" or "erratad" (like the V-stat, the Divine Challenge/Sanction kerfluffle, the implement system etc.) The builds themselves seem to be pretty satisfactory all things considered.