I didnt think my post was worded in a way to cause such strong feelings, however this subject is stirring enotions, so let me clarify.
The context I was endeavouring to provide wasn't about why UTMB is seen to be problematic but rather why Killian and Zach likely didn't list their reasons for considering a boycott.
I was speculating as to what they were thinking when they wrote the letter (I neither know them, nor received the letter. I'm just a fan) and so I'll continue speculating on why they are considering boycotting UTMB. These speculations are based on general concerns in the community plus the causes that they have publicly spoken about.
* The involvement of Ironman in UTMB and the affect that is having on the landscape of trail races. This has been covered in this thread already but if you want to read up more I'd start by googling the controversy surrounding the new Whistler by UTMB race.
* The firing of Corrine Malcolm who is widely liked within the trail running community and who is by far the best commentator on the UTMB English language stream. I can imagine they would be disappointed in this because they personally like her, but also because it will greatly dimish the UTMB experience for fans at home.
* (Possibly contrary to what Sage suggested earlier) The entry process for UTMB is not ideal for all "common runners." Lottery based entry systems are misaligned with efforts to create more diverse starting lines because they reward people who have the time and money to enter more qualifying races. This isn't ideal for getting more women or people of colour onto the starting line of UTMB. This is particularly problematic when combined with the point about the partnership with Ironman and the gobbling up of local races under the UTMB brand, as this has often (always?) lead to an increase in entry fees.
* The title sponsor for UTMB in 2023 was a car manufacturer called Dacia. Killian is quite open about his concern for the environment and his desire to limit air travel to attend races. I'm confident he'd find it incongruous to have a car manufacturer sponsor an event whose livelihood is dependant on the ongoing health of the environment.
This is a starter pack of reasons why people in the trail community are concerned about the direction UTMB is taking. Killian and Zach have been active in public discussions of these concerns. If you're keen to learn more, Google is your friend!