@nprofile1q... I also see it that way (and seen it like that back then).
AIX and HP-UX are really oldschool and old UNIX systems ... with all PITA that comes with it - manual package management, patches and fixes that often break each other ... and latest version of the patch does not always work and breaks the stuff - compatibility lists ... yeah you name it.
Solaris on the other hand - specially after OpenSolaris started to gain traction - with new pkg(8) tool by Ian Murdock (yes the one from Debian) ... and ZFS and ZFS Boot Environments and Comstar and Crossbow and Solaris Kernel Zones (with live migration) - yes - Solaris was kinda fresh - very limited amount of software in the Solaris pkg(8) repos - so if anyone wanted to do something there - then PKGSRC was needed. Later SmartOS even maintained the binary PKGSRC repo for Solaris systems ... not sure if its still the case.