I don't think they wanted to jump into a new entry so soon since, generally, new SMT titles are a pretty big thing. It may not be their most popular franchise, but it is their flagship, and releasing another main game so soon after 4 probably wouldn't work out because, if they did, they would have to give it the quality and size of another main SMT game (moreso if it's on a home console). That's a lot of money that they probably don't have.
Generally they can go about this by making smaller titles in the interim, but since then there hasn't been much, and similarly they've been working on other pretty large projects in the time since (ie. P5, Etrian Odyssey remakes). Ultimately, they probably can't cover the time and money it would take to create a new full-scaled SMT title right now. Not to mention that people would be skeptical since it'd be coming so close on the heels of its predecessor.