I think the big reason was absolutely the lack of content. Right now, it's a FULL priced game with practically nothing. No Arcade mode, even. Just online, with 2-player lobbies and a suck ass Survival mode. Oh, and Training mode.
Games like SSB and MKand even Narutocan sell a bunch of copies to casual players, who make up the majority of sales, because they actually have a bunch of single player content for them to dive into. There's none of that in SFV at this moment.
Edit: Not to mention the whole Fight Money and currency element operates like a F2P game... in a full priced title. It's not a good look.