It's often been speculated that her tremendous wealth was a main attraction to Alfred. He may have finally been wanting to settle down--Queen Victoria had been at him for almost a decade. There could've also been some competition factors at work--QV worried Affie was attracted to Alix (who was being neglected by Bertie) and if you couldn't have the most beautiful princess, have the wealthiest; also Bertie never stood up to QV and Affie apparently enjoyed rebelling at this period, something his brother would never dare. It couldn't be easy being the 'Spare'.
As for Marie, Affie was at the peak of his looks then and cut a dashing image in his naval uniform. Marie probably hadn't had too much interaction with eligible males at that point either. I think there was also some scandal about her and an aide/footman/soldier someone but can't remember where I read it.
QV was pretty prescient in her foreboding towards marriages of her family with the Romanovs--they all ended in a disaster of some sort. Her aunt Julie (GDss Constantine) ended up divorced. Ella ended up in a complex (to say the least) childless marriage that was the subject of gossip for decades with a husband assassinated and herself executed. Alix had a happy marriage but her position as Empress doomed her. Affie & Marie weren't happy for very long. Even Ducky's marriage to Kyril ended in bitterness.
