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.

Due to the Switch? So its not the ever growing mobile gaming?
Censorship will play no role in in sales anywhere.

PS5 will sale weaker in Japan due to not catering to their lifestyles. That's it.

Regardless of the mobile trend, the Switch will still affect sales as there would be even less reason to buy a PS5 for those that do still want more traditional gaming experiences.

I think it mostly has to do with the games themselves and how these appeal to the Japanese market. You do have some outliers like Call of Duty that are starting to do surprisingly well, but in it's totality the games Japanese people play are either Arcadey or RPG(like). If you can appeal with games like this, in tandem with a versatile device or console that conforms to the lifestyles of these people, you can sell well. Do I think Sony, with its action oriented cinematic games and a big black powerful box can do well in this market? No. Sony's eyes are fixed to the west.

What do you mean "regardless of mobile"? Its mobile that has the Japanese console market where its at. The Switch was created as result of that mobile trend. Dont just brush that aside. SONY is going to create a western focused console that will further distance themselves from the Japanese consumers. You really want Switch to be the answer but its not. 

Nail on the head.

Their exclusives nor the form factor of the console appeal to the Japanese market anymore.  Most of Sony's success there for the PS4 has been from 3rd party efforts.  With the PS5, most of the Japanese focused 3rd party games will also be on Switch.  The big difference for them is that the PS5 can only play on a TV natively while the Switch can do that and serve as a portable gaming device.  Also Nintendo's 1st party games actually sell in Japan as well.

The mobile market also plays a huge impact, and is another big factor.  This is a given for any dedicated gaming device however so it goes without saying.  With that in mind, there are more than enough gamers that want more traditional jrpg experiences and the like to maintain a very healthy presence when handled correctly.  The PS5 is heading in a totally different direction however and the Switch already had that sector of the market on lock.  If Nintendo decides to make a Switch 2 (they will), this trend will likely continue.

You seem to be confused, I never stated the Switch was the answer.  It is a big reason, but not the answer.  It seems more to me like maybe some would prefer to believe that the Switch cannot be a factor?

I don't think  it's a "big" reason. The big reason is PS5 will not be designed around Japanese sensibilities. I do agree with the prediction though. 

the switch is literally the mobile trend

Depends how much worst you are talking about. PS4 has gotten just about every big game available, and reaching 10 million is still questionable. I honestly can’t predict where it will go