I don't know if they set out to remove non-standard or they just felt they didn't need to support it. I would also imagine supporting it would set a dangerous precedent as non-standard rotations in other jobs would also have to be supported lest the community accuse SE of favoring Yoshi P's favorite job.

Effectively keeping non-standard alive is asking to take it into consideration when doing BLM job design for DT. It's far easier to just support the official rotation rather than figure out how to get both to work and be balanced. If another non-standard rotation is found, I'm sure they're fine with you using it if it's within balance bounds. And if you do, yay! Go use it.

For me, I'd rather they spend the time updating another job than trying to find a way to preserve non-standard for BLM.