I think Namco just wants to keep a
really tight leash over Tales Studio and the franchise when they can.
I'm not quite sure just how much control the company has over its own operations, but I'm going to assume Tales Studio gets little (if any) freedom of choice over which games they make will be localized, and who can localize them.
Maybe they're afraid that if they let Tales Studio work with another publisher to localize a Tales game outside of Japan instead of Namco handling it, the new publisher will manage to get a hit. And then Namco has to share the profits, and/or Tales Studio will make a push to work with other publishers outside of Japan for their games after they succeeded where Namco failed (and if push comes to shove, a "worst-case scenario" happens where a bunch of people leave to form an independent company or go to develop for someone else). That or the publisher might have some other kind of influence on Tales Studio Namco doesn't want showing up. I'd assume quality would be an issue, but I think some perfectly competent publishers have made attempts to no avail, so I dunno.
But then again several Tales games haven't done so hot and Tales Studio doesn't seem to care as far as I can tell, so eh. Just my crazy conspiracy theory.