While I do agree that unity is the way to go in the fight for rights, I can understand why one would want to separate the T from the LGB. It's an issue of consistency - L, G, and B all describe sexuality, while T describes gender. The two are related, but ultimately separate concepts - one does not inform the other, and grouping them can hypothetically lead ignorant people to think that they are directly related, which could hypothetically lead to non-straight cisfolk experiencing more oppression than they would have otherwise experienced due to the perceived association with transfolk, as non-conforming sexuality is more generally accepted today than non-conforming gender.

That being said, it's all hypothetical, and what matters is the reality that people from all spectra of nonconformity are regularly oppressed, and in many places, the oppressors treat anyone LGBT+ with the same disdain. So grouping them is vital for the sake of the most oppressed.