"88 politicians or candidates were killed between September 2020 and the
June 2021 Mexican legislative election.
[6]"
en.m.wikipedia.org
This is probably the reason.
The cartels kill all threats to themselves within Mexico, and all politicians in Mexico are suspected to be installed by cartels or paid off by the cartels.
Even to mostly honest politicians, if given the choice to do what you're told and you get well paid.... or you and your family are hideously murdered... is an easy choice to make.
Some don't even need the death threat.