Maybe they've just realised something:

"We've gone as far as we can with making unique, stand-alone games. The worlds and characters are getting less unique, more clichéd each time. Instead of churning out a formulaic 'new epic adventure' every two years, let's instead focus on the good work we've already done by expanding the ideas and worlds that people are familiar with and want to know more about".

That could be part of the reason for what they're doing. After all, FFT, FFTA, FFXII and Vagrant Story all overlap somewhat.