For greed! This has nothing to do with network security.
This is guys who even if the sat was worth $1000, would say that the minimum value of a transaction should be 1 sat.
If it's greed then they have a weird way to display it. OK, they want the minimum fee for transactions to be accepted in blocks to be 1 sat/vByte. Fine. However, others obviously don't agree and that's why they decided to accept transactions paying much less. The best they can do now is take what they can, be it an $1000 or $200 worth of BTC.