Nothing except user experience.

The Push lets you turn physical knobs to adjust sound parameters, and the pressure sensitive pads let you play drums / piano / whatever softly to loudly. The external sound interface is very low latency so playing a guitar along with sequenced music feels natural and records in time. And of course it’s more durable for live performances.

But technically, with the right software, you could produce the same music on an iPad. There are step sequencers and Ableton controllers in the App Store. They’re just much less satisfying / expressive / empowering to use.

It has buttons you can push! Seriously, for an instrument, the feel of the buttons is kind of a big deal.

This is meant for live "playing" as well as recording, so the feel of the buttons rather than a touch screen is important.

You should check out videos of people using the original push as a live instrument - it can take serious talent.

It's a highly optimized and specialized user interface for music production and live performance. It's tactile, the pads are pressure sensitive, and it comes with a high-quality audio interface for other studio gear. It's probably sturdier than an iPad, and it's user repairable.

Didn't pressure sensitive touch get removed from iDevices? Velocity sensing is a big deal with percussion pads.

It can upgrade or replace CPU, RAM, SSD and battery.

It has physical controls to let you develop muscle memory