Don't forget that Famitsu was also basically Nintendo-exclusive for a long time (indeed, for its first 10 years of publication it went by its un-abbreviated name of Famicom Tsuushin.)
I don't know why Nintendo didn't set up a publishing division and enter the magazine market earlier, but the earliest Famicom publications, Family Computer Magazine and Marukatsu Famicom, both were high quality publications right from the start, loaded with the same professional illustrations, maps, quality screenshots, etc. that one would expect from an official mag. Since most Japanese gaming mags prior to the PlayStation focused on Nintendo systems anyway (Sega wasn't really a contender in Japan until the Saturn), maybe Nintendo just didn't see the need to make their own. The difference in quality between early Nintendo Powers and the competing American mags at the time was pretty stark, but it's hard to imagine how an official Japanese mag would have improved upon what was already being published there.