“What reasons?“ might be a better phrasing of that question.
The greater geopolitical battle between PRC and USA - Pound pretty much stated that he expects the USA to boycott, or the PRC to ban the USA entry under various pretenses.
Covid-19 - Some countries still blame China for the virus, and may not be excited to attend the Games. Plus, China’s poor early response to Covid-19 might turn more athletes/NOCs off from attending.
Uyghur Discrimination - Human Rights Groups are campaigning hard for a boycott here.
Hong Kong - The PRC’s new authoritarian crackdown won’t play well in the West. I believe Canadian diplomats already suggested a boycott in response to this.
Border Conflict with India - India’s allies might refuse to attend the Games as a show of support for India should that conflict flare up again.
I’m sure you can add classic PRC issues like general human rights, Tibet, Republic of China v. People’s Republic of China, etc. to that list as well.