It is hard to pin down an exact reason. Maybe it was too similar to another app icon or simply new users didn’t like it as much.
You can run A/B tests in the store listings, it’s a hidden menu. Click Store Listing and beneath that, you should see experiments. You can run up to 4 icons (for example) and see which gets more downloads. This also works with the promotional video, screenshots, short app listing and full app listing.
There are a lot of customization choices as well.
I would make a few iterations of the logo, with different elements removed. I think the new one is a little too busy. try removing the phone on the left and right and see how users respond.