Dark Angels get the fancy Land Speeders, Deathwing Knights which add a LOT of options to Terminators, Ravenwing which add a lot of options to Bikes, two unique flyers, unique detachments based around their 1st and 2nd Companies, rerollable 2+ cover saves, artifacts that just wreck Chaos face for being Chaos which is the best representation of tabletop racism ever and still makes me, a Chaos player, cry. That's a lot of stuff to set you apart from vanilla, and a lot of it is competitive, not counting Characters.
The entire Imperium has lost technology between the foundation of the Legions and the present. Relics from way back when are called relics for a reason. Treated with unbelievable reverence or hoarded for the Grand Poobah to use once in a millennia. Ravenwing Master's Jetbike is a prime example. Every Dark Angel, every Space Marine, could have had access to jetbikes and in 30k it's a unit that is very powerful, but 10k years later, a Jetbike is so rare, only one guy can ride the ones they have left. Why shouldn't that be the same with the plasma weapons like Lion's Roar (I think that's the right one...)? There's an old fashioned safe plasma relic.
To directly answer your question about Centurions and other units only in vanilla, a good fluff excuse is that the Chapters with their own books are so steeped in tradition that they don't readily accept new wargear like the 'boring' line Chapters. Dark Angels and the rest are considered very old fashioned and pretty reclusive groups, so they don't line up to share and invite others in to share with them. To answer it from a game perspective, there has to be a reason to take vanilla over these single book Chapters, so the Dark Angel book gets all the stuff they do, vanilla gets their unique stuff to entice people there, Blood Angels get their unique stuff and so on. If they just gave Dark Angels everything and a plasma bonus, who in their right mind would buy the vanilla codex? They'd run Dark Angels that look like Ultramarines or some such thing. Shouldn't have one book be overpowered to that degree, just have them different.