I only just found this thread, or probably would have given my two knuts earlier but here goes thoughts at 3am, which are probably not all that clear. In fact rereading it just now after typing it, its totally jumbled but Imma post anyway lol.

I think the biggest factor from an IC perspective is safety. We can all agree that younger characters, regardless of what kind of beast, being, witch, wizard, whatever they are, in theory should be less in control of their abilities, and while in canon their are laws about not being able to hunt Vampires and all that sort of thing, we have to remember that Vampires are Non-Wizard Part-Humans (Magical Beings who are considered to be neither a wizard, nor truly Human), and not witches or wizards, which is what Hogwarts is for.

Now a possible exception to that might be a part-vampire, which we know is possible since theres Lorcan D'Eath, but what we don't know is if Lorcan had any magical ability to speak of at all. Who is to say that vampires are ABLE to do magic? That's a factor, since after all, it is a magic school and it'd be pretty important to be capable of magic in order to actually attend. Taking your debate approach we could also say that part-Veela who we know CAN do magic might indicate that the same is possible with vamps but unfortunately until more information comes from JKR we don't know for sure. Either way, in the books there aren't Veela at Hogwarts, but there are at Beauxbatons and in a way that supports my personal thoughts on vampires in that:

due to safety of students, part-vampires are not permitted to enrol at Hogwarts.

and additionally, they are in canon from Eastern Europe so if any of them actually are magical and needing education, it stands to reason they're probably at Durmstrang. (but don't get me started on the whole concept of going to Hogwarts when you don't live anywhere near the school and haven't actually worked out a legit reason for your charrie to be there hehe)

It seems that JKR's vampires are inspired by those of myth and legend, with the blood and the garlic and all of that, and they certainly aren't harmless. Personally the parents of my characters probably wouldn't want their children attending school with someone that could lose control and attack them.

Then we have the OOC perspective which I think is more relevant to our RP on SS. You mentioned about them being powerful and it being unfair to other players, and you mentioned the blood-drinking and SS being family friendly, but honestly I don't think those are the underlying reasons. They contribute, sure, but I think there is another factor.

Because we know so little about HP vampires (aside from tiny hints, and speculation regarding myth and legend) and because in the last.... 8 or so years there has been an EXPLOSION of vampire related books, television, and movies, I think the main consideration is that we aren't a crossover RP and we want our RP to be as in line with canon as possible, and most people (not all, definitely not all) would likely get their inspiration for vampire stuff from popular fiction. I'm not just talking about sparkling vegetarian vampires, I mean there's also Vampire Diaries and True Blood plus a whole myriad of other popular media where vamps make an appearance. While its fine to be inspired or whatever, the fact is the lore surrounding all this media is so varied and different, it'd be nearly impossible for the thousands who have and will RP on SS to be on the same page with it, and anything that is approved will become SS canon and we don't want to be using canon that has the potential conflict HP canon AND with the head canon of other potential Vamp RPers.

"Well my vampire can fly!"

"Well mine has to be invited inside or he can't come in."

"Mine gets burned by crosses."

"Mine sparkles and is super strong."

"Mine is a vegetarian." and so on.

Additionally, I really don't think allowing vampires at Hogwarts would add to the RP in a way that is conducive to everyone having fun. Special charries require permission so that it can be monitored in a way that facilitates everyone's enjoyment, and doesn't make anyone uncomfortable or forced to participate with someone else's canon IC.

Once we allow ONE person to do it, it opens the door for everyone, and at that point its kind of not fair to say this person can and this one can't. Its overall just easier to have a blanket no. Then the ooc of it can be explained away IC easily enough.

The only way vampires would ever potentially make an appearance would be via an arranged plot, one that can be controlled and monitored.

SS isn't really a dark-inspired RPG and there are plenty of places out there where a person can go RP if they want to explore that side of the wizarding world, and I think vampires and vampire lore tend to fall under that umbrella.

TLDR? In summary, the idea of it 'always being that way' is actually because there are reasons its always been that way. And there are IC and OOC concerns surrounding vampires in our RP, particularly the school.