Others have touched upon this, but the reason why T is grouped with LGB is that we are all perceived as not being heteronormative by society, and we are all subject to discrimination (and worse) based upong that perception.
As has been mentioned, femmy gay men and butch lesbian women are often perceived as being gender non-conformists (or even as being trans in some cases), and they do get discriminated against on that basis. Similarly, trans people are often perceived as being gay. There are numerous instances of trans women being assaulted by people calling them faggots and the like.
So ultimately, we are natural allies (as are the genderqueer, queer, and questioning people that are sometimes included in the alphabet soup). And yes, it'd be simpler to just call us all queer, but some LGBTGQIQ people object to being labeled as "queer".