Because pkexec has the same problems as sudo: it's a SUID binary. As Lennart says, the goal is to eventually get rid of SUID binaries altogether, as they are an

Because pkexec has the same problems as sudo: it's a SUID binary. As Lennart says, the goal is to eventually get rid of SUID binaries altogether, as they are an inherent security risk. Replacing sudo with pkexec would not change much. In fact, pkexec has had its fair share of local root exploits over the past few years.