Because in Japan the game was HUGE. "Theme of Love" is now used in early Japanese music classes for practice/performance, for example. Also, I remember reading an interview with Tetsuya Nomura in EGM about how even after FFV and FFVI was released, fans of the series were still talking about the characters of FFIV more. One of his goals while working on FFVII was to create more memorable characters.

I wish I could provide links for this information, but those are two major examples I can think of on why FFIV got such high priority to get a real makeover, instead of just another port.