https://www.thedrive.com/news/heres-why-the-automatic-gr-corolla-doesnt-use-a-dual-clutch-transmission said:
“So at first, considering the power and performance from a standing start, the torque converter is working very well. And also the serious driving around the road, with speed conditions, torque converter is also doing a very good job,” GR Corolla Chief Engineer Naoyuki Sakamoto told The Drive.
“Then after rolling out, lockup engaged—so after that, the torque converter doesn’t slip. So in that case, there’s no difference between the DCT and automatic transmission. Both [transmissions are] shifter-changing, releasing the clutch and grabbing the clutch, so they’re almost the same. So as a result, we’re choosing the automatic transmission. We don’t have to use a dual-clutch transmission.”
It seems that, at least according to Toyota, there wasn’t much of a performance gain to be had by switching to a more complex and more expensive DCT. Toyota knows that DCTs have their advantages, but they apparently aren’t attractive enough to warrant the switch.
“Of course, there are pros and cons. Sometimes a DCT is better, sometimes AT is better. That’s true. But we have a long history of improving the automatic transmission,” Sakamoto said. “We want to take advantage of our capability, so that’s why we’re focusing on the automatic transmission. We understand a DCT is better in some ways, like weight, but still, we’ve been able to improve the technology of the automatic transmission through motorsports.”