Yeah, in the manga Rose was a one-shot character who I don’t think ever appears again after the first story arc. She’s meant to make a statement about how faith can turn into stubborn, blind stupidity. I rather liked her that way, myself, but never expected any more from her.
The anime only re-used Rose since at the point in production when they had to start making up new story material, it would be much easier on the animators if they could just re-use characters who already had model sheets prepared. The alternative would’ve been designing whole new characters, which can be a stressful prospect when a show’s already at mid-season.
Original series was just slower and more detailed, taking time to get you attached to characters and make you wonder about their quest.
A lot of that is because the “adapted” half of FMA is nearly a panel-for-panel adaptation. They were clearly trying to stretch out the manga material produced to date as far as possible. Some manga arcs go into multiple episodes and there’s one (really silly) episode based on omake material.
Brotherhood is IIRC trying to speed through the same material as quick as possible since it’s already been animated once before, but can’t be disregarded entirely if you’re trying to do a more faithful adaptation of the manga.
As a final note, though, bear in mind that FMA2’s production team significantly differs from the original’s. In particular, FMA was directly by the talented vet Seiji Mizushima while Brotherhood is being handled by someone who I believe is a relative newcomer.
It really seemed like brotherhood was assuming people watched the old series and speeding along to get to “something.”
There’s an absolute ton of characters from the manga who ended up never appearing in the original animation, so I suspect Brotherhood is just trying to law groundwork quickly before letting them show up. I would actually bet the opportunity to merchandise the manga-only characters is the main reason why Brotherhood is even being bothered with.
I also imagine it’ll slow down a bit when it gets to parts of the story the anime inadvertently changed early on, to emphasize that it’s setting up Event A correctly this time so Event B can happen as intended later on down the line.