Curious, why can't you use oestradiol gel on buttocks?

[AlwaysQuestions]

I know you can put hrt patches on your bum or hips but the gel has to be applied to outer arms or inner thighs. Does anyone know why this is, is it dangerous to use gel on your bum cheeks?

From what I've read it's to do with thickness of skin and blood supply to those areas being more conducive to absorption into bloodstream.