Why did they remove regen from vampires?

Like, I know that they still regain HP from biting people, which is nice, but I'm still a little disappointed that flat regen got removed from ALL vampires. For vampire spawn it makes sense gameplay-wise. (Running regen that can be disabled for a lot of monsters is a DM nightmare). But for full vampires it's just a little disappointing. (The lore of vampire's regenerating is that they kind of heal instantly if they HAVE drank blood recently, not only if they directly drink it). What do you think?

There were game design issues where Regeneration could make a fight way too easy or way to hard depending on how much damage the party can deal each round. Nixing Regeneration helps ensure more consistent difficulty.

Quote from Maruntoryx >>

There were game design issues where Regeneration could make a fight way too easy or way to hard depending on how much damage the party can deal each round. Nixing Regeneration helps ensure more consistent difficulty.

I don't know. Trolls and Revenants still had regen tho, so clearly it wasn't bad for all monsters.

Yeah, but regen is basically the defining trait of trolls.  Also, trolls don't have an special movement options.  Vampires are hyper-mobile.  With regen they can easily rush in, do some damage, then escape only to come back a minute later after getting back up to full health and hit the party again.  And again.  And again.  I've been on the receiving end of this sort of tactic before and it's pretty much the opposite of fun.

I understand the pain of masters due to the hit-and-run-deflect style of combat, but instead of stupidly cutting regeneration, the developers should have tied it to resource use or legendary actions.

Vampires already have a flavorful (pun intended) mechanism for regaining hit points - their bite attack.  The bite is an active heal - it requires the vampire spend a resource (its action), gives the players the ability respond if they are able to interfere with an attack hitting, and requires set-up by the vampire, making avoiding the bite a bit of a puzzle and choice by players.  It also fits the flavor best - the vampires need blood in order to heal (common in a lot of vampire lore) and vampires are often depicted as very active participants in combat, fast, brutal, and utilizing some tactical decision making. 

Regeneration is passive healing - it simply happens.  Far less interesting a mechanic.  Sometimes that makes sense - for trolls, the challenge is making the fight a damage per round check, where outperforming the troll’s regeneration is the name of the game.  This often makes for monsters that feel more like busywork than an actual challenge though.  Passivity rarely leads to interest. 

Rather than have vampires have two sources of healing that could result in drawn out, repetitively dull fights, they picked the one that made sense and cut the other.  From a game design stance, it absolutely was the right choice.