This is because as time since last block increases, fees in mempool increase. So it becomes more profitable to mine the larger the mempool gets
By the time 60 or 90 minutes has gone by since the last block, the mempool would be so large with fees that 100% of hashpower should be online trying to get them
Conversely, after a block is mined the mempool should be empty and without a subsidy it may not be profitable to mine until mempool reaches a certain size.
So there will be pressure for low hash rate before 10 minutes and high hash rate after 10 minutes, giving a much more consistent 10 minute block time.
