It started because in the past most trans people were homosexual (for example, a MTF who is attracted to men). Now that it changed, LGB still support the T because they are being brainwashed by the media or are scared of speaking against it. For many gay people, LGBT spaces are the only places we can receive help/support or find other people like ourselves and not feel so alone, so the thought of being kicked out of them is scary. Besides, if you get branded a transphobe you are fucked, you will be doxxed, treated like shit and maybe even lose your job and be physically assaulted.