We never get any explanation for what happened during the period between Jessica Jones and Luke Cage, but the best explanation for why Cage was working at Pops is that he had gone into hiding after the Kilgrave affair.
Remember that Cage was a fugitive from the law. He had sunk all of his money into opening his bar and trying to keep a "low profile". Also, when he opened the bar, Reva was still alive, so any paperwork they needed, she could have handled to keep his name off of it.
His bar explodes after Kilgrave had him blow the bar up. Even though he was being mind controlled, that would probably make getting an insurance payout complicated. On top of that, he was becoming involved in high-profile incidents around Hells Kitchen, the kind of thing he was trying to avoid.
Instead, Cage appears to have decided to abandon Hells Kitchen and go back to Harlem. He wouldn't want to take a regular job, lease an apartment, etc. because that would put him "in the system" again, and he was trying to disappear. So he took whatever jobs would pay him under the table, and an apartment that he paid for in cash.