Because if the microwave can do it in 10-15 min, the toaster oven is going to take 40-60 min or worse.

Waiting 10-15 min to reheat something evenly is doable for me. I can make a side salad, toast some bread, empty the dishwasher or whatever while I wait.

Waiting 40-60+ min requires real advance planning, which is often just not feasible.

If you've ever used a toaster oven to warm up a frozen personal-size lasagna, sometimes it can take as long as an hour and a half.