LogFAQs > #985062393

Xillia 1 was rushed out in a trimmed down state lacking many forms of optional content traditional to Tales games, and introduced a whole other world in the last 10% of the game in a very limited capacity, so debatably a second game was justified to actually complete the experience as well as flesh the new setting out. Additionally they got to tell a new story while largely just reusing the first game's assets, very affordable.

Symphonia 2 was a lazy cash-grab brought upon by the first game bringing the franchise mainstream popularity.

As for the other games not getting sequels, Berseria is a prequel and thus technically already has a sequel, Vesperia was long as fuck and kinda sputtering out as-is (the game hits a climax then just goes on and on and on like some daytime TV drama) and then got a rerelease with a bunch more content, and Abyss ended with an intentional ambiguity that a sequel would kill flat.