Lopez began his hosting gig in November of 2009, with an 11 p.m. slot. Once Conan O'Brien moved to TBS with Conan, Lopez was pushed back to midnight, which didn't help matters in terms of his viewership.
Reportedly, the network wanted to avoid this choice as long as possible. In the past, TBS claimed to admire Lopez's comedic talents and his work ethic and attitude. Notably, Lopez expressed no malice or disappointment publicly after the network pushed back his time slot.
Source: Hollywood Reporter