The puzzles had stalled because of the bots, but once it was made public Mara via Wp, the search resumed. So, it was just a matter of low time.
I think they stalled because, computational power was too slow and expensive for the puzzle rewards, given that they were 1/10th of what they are now. And the BTC price was much lower as well, so anyone going after the unsolved puzzles would spend magnitudes of order more money than what the puzzle rewards were worth at the time.
After the unsolved puzzle rewards became 10x and the price of BTC price went way up, and GPU's became much faster and cheaper to rent, it was now possible to solve a puzzle and come out ahead.
Didn't have anything to do with bots, they have been around since the early days of Bitcoin. Nothing to do with MARA either, they just started their Slipstream service not very long ago.