I have no real worth in saying this, but I personally believe it's because Soulcalibur isn't as big as a priority for Namco than Tekken. Having a game be released on console only means that you can just release the game and get your profits immediately as opposed to the constant upkeep of arcade machines and yearly revisions. You can just put the game out, maybe patch out some of the more egregious broken shit and you're pretty much done. Given what I've read about Project Soul repeatedly getting disbanded to work on Tekken games, I wouldn't be surprised to find out that the reason Soulcalibur games are console exclusive is to funnel more money into Tekken's expensive arcade cycle.
I believe the other reason can be found in this very thread. Like it or not, Soulcalibur is mainly a casual fighting series game. SC3 was so unbalanced that it (combined with a very late, very limited release of SC3:AE during the final death throes of the US arcade market) killed the US tournament scene. SCIV had the Doom Combo, SCV has Viola, and SCII HD is a shitty port job. It also doesn't help that the games to address these problems (SC3:AE, SC:DB, and LS) were not updated version of the main games, but weird (and suspiciously inexpensive) side-projects. There are more people here that would rather have the game come out the gate sooner than better, and I can't blame them, but money talks. And the money cares more about the shiny extras than the game itself.