There is no love lost between Johnson and Cameron - they hate each others guts.
Johnson would not hesitate to throw Cameron under the wheels of the bus, except the collateral damage would hurt the Conservative Party as a whole.
What we are seeing and experiencing is the result of winning a large majority. It happened to Thatcher’s Government, and then Blair’s. The politicians feel entitled and untouchable with the country backing them so conclusively. It makes the cocky and arrogant.