A number of reasons (I do this for a living):
- Nobody can get insurance to cover covid and committing to non refundable production costs then having to cancel would spell financial ruin
- international travel for artists is expensive and for American acts is generally only economically viable to travel across the Atlantic for UK+European shows. Quarantine and varying levels of lockdown are making summer and autumn planning hard at the moment.
- sales and marketing windows are now too tight to turn it all around
Nobody really has much/any inside knowledge, pulling large events in 2021 is purely a pragmatic business decision.
Smaller gigs are less risky and more agile so are far easier to manage in the time frames we’ve got.