I think it's because of the reward system changes. EV of rewards dropped so players started optimizing rentals more, be more selective or just gave up playing an account with rented cards. I know at least 2 champion accounts that stopped renting because it started to generate a loss.

In the beginning, for me the rule of thumb for most of the cards was "rental fee below 4-5% ROI is ok", right now it's more like "rental fee below 2-3% ROI is ok".

During the last month, every time my contract ends, instead of extending the same one, I can choose another one, which is usually 20-40% cheaper.

PS Does anyone need cheap Harpy? Here you go. Basically you can have maxed card for 180 days in the price of 1BCX: