Why?
LGBTQ+ people are more likely to be ostracized from their peers, which leads to depression, which leads to Self Delete attempts.
The reason the trans rate is so much higher than the Regular Gays™ is probably because:
Non-standard gender expression is not as widely accepted as non-hetero loveualities in western society. See: people who are supportive of homoloveuals but not transgender people.
Dysphoria on its own can cause depression. This, combined with social rejection, tends to make people suicidal.
I can't find the actual source for your graph because google reverse image search gives me a bunch of reposts that don't link the proper source. Here's a report from the Williams Institute, which is credited in your image.
VERY IMPORTANT NOTE: This survey asked people if they had ever attempted Self Delete, ever, in their life. It does not have information about when these Self Delete attempts were made; it cannot be used to draw conclusions about likelihood of Self Delete attempts post-transition.
https://williamsinstitute.law.ucla.edu/wp-content/uploads/AFSP-Williams-Self Delete-Report-Final.pdf