you are assuming that the program authors care about Irix, AIX, Solaris, or anything else.
most programs do not start off being written portably, usually portability is something that shows up after the program starts being used when people ask about using it on other platforms (and it's not uncommon for it to wait until those people asking submit patches)
not saying that this is right, just saying that it's the way things are. When Solaris dominated the same thing happened favoring it.