Honestly I think it’s just that you can just order them over the internet hassle free and probably cheaper
If I'm not mistaken you have your choice of Chinese semaglutide (Ozempic), tirzepatide (Mounjaro), and retatrutide from these peptide sites. I think for example tirzepatide has some benefits over the retatrutide wrt to heart health or whatever so they have different profiles and it's claimed reta is just overall superior so it seems it most popular for that reason.
I don't know about stats but I assume most average, non-bodybuilding folks don't want to mess with the Chinese peptides and don't care it's many times cheaper and would rather pay for brand name Ozempic, Mounjaro etc. I told my aunt her husband could get his Ozempic much cheaper off the net but forget about it
