I can think of a few reasons:

1. Android is open source

That makes easier to analyze its workings and identify vulnerable code.

2. It's more used

More people use Android than iOS, so the resulting malware would infect more people.

3. Emulators are more accessible

Android emulators are more accessible. That makes testing different Android versions way easier.

4. Sideload is very easy

As [Robert properly commented][1], it's easy to send someone a random APK and they will probably install.

5. Custom ROMs

There are hundreds of custom ROMs available, and some of them disable security features. They are mostly intended for development use, but some people use it as default ROM.


  [1]: https://security.stackexchange.com/questions/249291/why-are-there-more-research-papers-on-android-malware-than-ios-malware/249292?noredirect=1#comment512982_249292