Just as the title says, for a few reasons. One is Sony's stance on censorship. Many Japanese devs, particularly smaller ones, cannot be bothered with it. While bigger games such as DMC5 remained untouched in Japan, other games most certainly are. Regardless of how you might feel about this censorship, it will impact sales in Japan. Especially considering that their biggest competitor, Nintendo, is not making the same restriction.
Which brings me to my next point, the dedicated home console is now even less relevant than it used to be in Japan due to the Switch. While the Switch is weaker, it is capable of running most Japanese games for years to come. The easy form factor of having both a portable and home console option in one package combined with not pushing cencorship will make it so there is little reason for Japanese gamers to invest in a PS5 unless they really care about specs (which most gamers there tend not to).
With the PS4, there was a reason for a Japanese gamer to get one alongside a 3DS if they wanted an option to play on a TV. With the Switch however, that need has greatly diminished as most games that are Japanese focused will no doubt also be coming to Switch. Many will come to PS5 as well to satisfy western jrpg fans, but will be dominated by their Switch counterparts in Japan (they will sell close to equal in the west, as both Switch and PS5 will have a big presence there).
I am not saying the PS5 will outright flop in Japan, but when these factors are considered, I do not see it even coming close to PS4 sales.