Easy: Namco likes the Tales series to sell well in Japan, and it did that on the PS2. Tales of Symphonia did not sell nearly as well on the Gamecube as it did on the PS2 in Japan, even with a long delay between the original and the port's release.
It's of worth to note that it wasn't until after Tales of Symphonia that we began to see Tales games released in North America frequently, or as frequently as a series can be released. It's of further worth that most of the projects released were for the PS2, though there was one PSP release and a GBA release, as well, and since then, we haven't seen the series except for upcoming games, two of which are on Nintendo platforms.
When you consider the development lifetime of a product, along with the userbase of the other consoles last generation, both in North America and in Japan, it's clear that it just wasn't worth the resources to port/make a second tales game for the Gamecube: In Japan, with the option between PS2 and GC, the PS2 won handily, and I believe in North America, sales of Tales games were about even for both platforms, though I haven't checked that. Furthermore, it's likely that the games you're complaining about were in development for the PS2 by the time Namco gained a complete grasp of Symphonia sales, decreasing the likelihood of the project switching development to a different platform.
As far as why they didn't port an older game over: Symphonia's sales took some time to accumulate. The game was released in August of 2004, if memory serves correct, and sales were steady up until Christmas, I think. I'd check, but I'd rather criticize you. Essentially, Namco wouldn't have great sales numbers for the game until January 2005, which, of course would be another holiday season where Gamecube sales were much lower than their competition. Essentially, to most developers, it seemed like everyone owned a PS2 by now, including owners of Gamecubes, and thus, they likely believed the PS2 to be the platform that could reach the most people, and that a GC port would take too much time and resources for too little return, considering most Tales fans would already have the game on the PS2.