Err... Remember... We're talking about the West here.
Only 4 Tales games were released in the West on Nintendo systems ever. Tales of Phantasia for the Super Nintendo (can't find any reliable numbers for this title), Tales of Symphonia on the Gamecube (which still sold better on the Gamecube as opposed to the HD remake on the PS3. The PS2 version never made it out of Japan), Tales of The Abyss (which outsold the PS2 port, but mainly because this version was released outside of Japan as opposed to the original game) and Tales of Symphonia: Dawn of the New World (which sold 200k in the US, but the PS3 version was never released outside of Japan).
Tales of Xillia still didn't outsell Tales of Symphonia, which was on the "flop" known as the Gamecube. There's overwhelming evidence that the Tales games do better on Nintendo systems in the West.
If this is about western releases, then your original post didn't make a ton of sense. Of the last four Tales games to come out on Nintendo systems, two of them came to the west, including the most recent one. That doesn't really fit with "Namco refuses to release this series on Nintendo systems". That's a better ratio than the most recent four Tales game released on non-Nintendo systems (though apparently Xilia 2 and Hearts PSV were announced for the west in April which will put those ahead.)
I'm also not sure how that is overwhelming evidence. Basically Symphonia did well 10 years ago. Since then compared to the PS3 and 360 releases tales did worse in the west on the "massive hit" known as the Wii, and slightly better on the 3DS. Although, the NPD numbers I can find are significantly different from the numbers VGC has, so this comparison might not even be useful.
The main thing they don't seem to like to do is release handheld games in the west. Seems like so far only one GBA, one PSP and one 3DS game have gotten full western releases. Also it doesn't look like any mobile games have made it over. Really, other than Graces they seem to treat Nintendo releases the same as others in terms of whether it comes to the west or not. I'd imagine that Vesperia PS3's success and Graces Wii's lack of it shifted their priorities from "let's release this in the west" to "let's release this on PS3". If Dawn of the New World had performed like the first Symphonia in the west, I'd imagine their plans might have been different.
You seem pretty confused.
We're talking about Tales games releasing on Nintendo systems in the Western hemisphere. They sell very very well on Nintendo systems outside of Japan. Tales of Xillia only sold 250k in the US and 120k in Europe. Meanwhile, Tales of Symphonia: Dawn of the New World sold 200k in the US and 40k in Europe. In comparison to Xillia, Tales of Symphonia: Dawn of the New World sold very well. Tales of Graces sold similar numbers. There's no evidence here that shows Tales games *don't* sell well on Nintendo systems. So, the question here is... Why not release them on Nintendo systems?
RPGs tend to sell very well on Nintendo systems in general. There's overwhelming evidence to prove this.