I’m not actually involved in the project, so I don’t have the details. If I understand correctly though, it’s because:
- There wasn’t a free pin until recently. (or there may not be a free pin even now?)
- Three-channel drivers weren’t supported by any firmware when the driver was created.
- Ramping is generally less smooth and more fiddly with more channels.
- It’s too late to change the driver.
- The extra parts could raise the price.
- Diminishing returns; going from 2 channels to 3 is a smaller upgrade than going from 1 to 2.
Basically, the extra complexity probably isn’t worth it right now.