Tales of Zestiria and Final Fantasy Type-O have just been announced for PC within the last few weeks. Valkyria Chronicles and a plethora of Final Fantasy ports made their way last year/this year. It is only a matter of time before XV is announced. 

Could this be because consoles are doing poorly in Japan, and Japanese developers are trying to gain some of their lost market on moderately capable PC's (these games aren't particularly demanding to play at 720p)? 

Yes. Though I appreciate SE's first step to put their games on PC and I hope this will be a real trend forward for them. FFXV on PC would be awesome.

nope they just take advantage of x86 as they want to release on everything like on vita and PS3 as well, many JRPGs are still exclusive or delayed.

Certainly, but it's not like there are new JRPG's being released on consoles to be ported to PC. Type-0 HD just released on PS4/XBO, and it is a huge hint that PC might get FInal Fantasy XV. As for Valkyria Chronicles, why port an old, somewhat, niche title to PC, if you don't think there will be a market for it? 

Meanwhile Zestiria is announced for PC and not XBO. 

Why isn't Zestiria coming to XBO then? 

because the Xbox One isnt popular in japan at all, most Japanese people own a PC 

Couldn't they get Western sales? 

Most none Japanese Gamers who are into JRPGs mostly play on Playstation, and the Xbox 1 isnt doing that much better in continantal europe

More like the old guard in Japan finally realized that a lot of people in the west play games on PC.

Generally speaking, the businessmen who run Japanese publishers aren't a very proactive group. They usually try to force the rest of the world to conform to their way of doing business rather than the other way around. I'm guessing someone pointed out, hey, you know, Steam has like 40 trillion members, maybe a few of those would like to play a Japanese game.

Personally looking forward to remasters of old classics like FFX or Kingdom Hearts for PC. Square enix has gotten good at PC now days. You can basically get every FF title from 1 to 8 on PC. Wish they would go full cycle since 20 years from now you wont be playing your PS3s or PS5s either for that matter, but your steam collection will always be there.