Mavren’s ability gives you a 1/1 Vampire creature token with lifelink whenever one or more of your nontoken Vampires attack. While one token doesn't seem like a huge deal, the incremental advantage adds up quickly. The added ability of lifelink on those tokens also makes it much harder for your opponent to wear you down over the course of a game.
It also helps that available to those Mono-White Vampire decks was Anointed Procession, a token-doubling Enchantment. Add to that early Mono-White Vampire decks often played Angel of Invention which pumps all of your other creatures by +1/+1, suddenly making those tokens suddenly twice as scary!
With Vampires being a popular tribe in Magic the Gathering, there's also a lot of casual demand for Mavren Fein. Vampires are a huge deal at the kitchen table, and Vampire Tribal Commander decks also want Mavren. A few select decks play the Dusk Apostle more than others, including decks led by Edgar Markov, Vona, Butcher of Magan, and Elenda, the Dusk Rose.
While Mono-White Vampires didn’t become a long-term contender, Mavren Fein, Dusk Apostle is still a good card. Especially in Commander where Vampires love creating tokens, the additional tokens from Mavren are a great complement to an already powerful tribe in the format. The only downside is that many Vampire decks don’t play White, but those that do are happy to have Mavren Fein around to help press the attack.