I think it might be because ICC and Cricket Australia want tickets revenue whereas BCCI can afford hosting IPL only on advertising
The same reason can be for Euro 2020 getting cancelled but premier leagues have continued without fans
In addition, it is one thing for a domestic league to function with some overseas players. An international tournament would require the permission of multiple governments, sporting bodies, different players, media people, staff etc and very possibly would require a lot more co-operation from a large group of individuals which means that the risk is higher.