Because there can only be one lead actor
Incorrect. As many people can submit for lead for a given show as want to (Martin Sheen and Rob Lowe both did for The West Wing
, for instance, or all four of the lead women on Desperate Housewives
Paul is definitely staying out of Cranston's way, though (partly also because he started out in Supporting in S1, when he was more of a supporting player, and people rarely jump categories).