Solaris runs on commodity x86-64 hardware, Oracle offers installers for download and a free license for development use, and there is an open source fork (Illumos) which is 99% compatible. Given all of this, it isn’t hard for open source developers to setup CI/dev/test environments to support Solaris x86-64 as part of their project. SPARC is harder (hardware availability), but if it builds on Solaris SPARC and tests fine on Solaris x86-64, it is
probably going to run on Solaris SPARC too. There are some potential gotchas around memory model differences and endianness, but one can just punt those issues until a user reports them, and then say “we can’t fix it unless you let us use your hardware”
AIX only runs on non-mainstream hardware (POWER), plus IBM isn’t as open as Oracle is with providing access to it. This makes it significantly harder to support than Solaris is
Added to that, Solaris and Linux are closer to begin with. Historically, Linux copied a number of its APIs from Solaris; AIX was far less of an influence. Years ago, I had to port some psuedoterminal management code from Linux to both Solaris and AIX. I only had to make some minor changes to the Linux code to make it work on Solaris; the changes I had to make to get it to work on AIX were much more extensive