Basically everyone was forced to start selling votes as the first big accounts who started were earning almost 2x more rewards. I think this started when curation changes occurred such as linear curve, I.e. a vote represented a fixed amount of rewards along with the change from 75% author and 25% curator to 50/50. Lasted for quite a long time, this was also why we created ocdb which worked as a more selective bid bot, it had a whitelist of users, only those could buy votes from us, they were also all profitable but the project itself didn't make any profit, which most other bid bots did.
Then we introduced the 25% downvote mana pool giving accounts 2.5 daily "free downvotes" because before that if you wanted to downvote it cost you regular voting mana meaning those downvoting forfeited curation returns.
A few projects then started downvoting bought votes, making them not profitable for the author and after a little bit of drama here and there everyone kind of agreed to switch to curation.
That sounds quite bad, not really tempting to join... I think the new mana for downvote made the big change otherwise you lose voting mana (hence rewards) for nothing beside fighting abuse, not sure many would have done so