Why is SotN the most complex of the metroidvanias available? I understand the three for the GBA being dumbed down a little, processing power is lacked and you only have 4 buttons to work with. But the 3 DSVanias are dumbed down as well? They hold your hand most of the way through and none of their equipment systems are as complex as Symphony's.
For one simple reason: Balance.
SotN's "gimmick" is the enormous amount of weapons. Dawn's gimmick is the enormous amounts of Souls. Ecclesia's gimmick is the amount of Glyphs (and you're dead wrong on this one because it's very close to SotN once you switch the word "Glyph" for "weapon").
SotN is not the "most complex". It simply gives you a lot of shit (many of which are almost duplicates or have little use before being discarded for better stuff) and cares little for balance, which is why Alucard can be made into a god relatively early in the game. Many choices != Good choices.
You said that the others don't have "complex equipment systems like SotN", and damn, I disagree so much. Soma and Shanoa's equipment systems come off as much more well-thought-out than Alucard, where you could basically slap anything on him in combination with anything else pretty much most of the time and he was still a murder machine. If you try to do that with Soma you risk being at a quite annoying disadvantage against some enemies, while in Ecclesia you risk dying over and over until you learn how certain glyphs work. That's not even getting to the Doppelganger soul, that allowed for multiple equip configurations, or the system for raising the power of certain attack elements that Ecclesia had.
Alucard didn't have souls to collect, didn't have glyphs, didn't have dual-item crashes or a very vast array of reusable magic attacks (not counting the Duplicator). Hell, SotN didn't even allow you to use consumables without having to equip them first. As I see it, SotN is hardly superior to the Metroidvanias coming after it on the gameplay/challenge aspect.
EDIT: I noticed now I'm coming off as a bit of a douche.
I'm not saying SotN is a bad game. It really is not. What I'm saying is that it's difficulty being all over the place is a result of the not-so-complex equipment system it has. You said you think it has the most complex system, and I'm pointing out that, to me, it really is not so complex. Just because it has a lot of stuff to collect and customize, doesn't mean it's actually "more intelligent" than the systems coming after it. While on SotN you'll get by just fine equipping basically anything together with anything, if you don't get how to balance yourself/make up for your strengths and weaknesses on the later games, difficulty can actually bite off your ass.