I didn't know that I shouldn't vote on posts older than 1 day. What is the reason for this?

There is basically a mechanics (curation curve) that lowers the rewards for late voters after 24H. It used to be 5 and 15 min :) . This is something that initially was implemented to incentivize content discovery and rewards the ones who discover it early and voted early, but in practice it was botted a lot and not achieved its goal. Then it was pushed to 24H, to keep at least some incentive for early content discovery, but remove the advantage that bots had voting in 5 min window, and level up the game for humans :)