Hi, cofounder of Simple here.

We require a smartphone for more than just mobile check deposit. We want people to have our app installed so we can use it for multifactor authentication, for example. We also want to do rich push notifications of account activity, and SMS (the only viable non-smartphone option for that) isn't a great experience.

Today, there's still a bunch of people who don't have smartphones. But the previous generation of iPhone is now basically "free" at some carriers, as are many Android phones. By the time we're off our invite list and open to anyone who wants to sign up, it's going to seem crazy to support anything that isn't a smartphone.