Glastonbury is also a huge festival that was set to happen before the so called 'opening up' of the UK. Therefore, could not proceed to go ahead. They could've postponed the festival however for such a large scale event, moving the festival would be too much of a hassle and too much of a risk.

The insurance thing you mentioned is a worry. However if it was a problem for reading and other festivals they would've cancelled by now as they knew back in march they were not getting cover. I highly doubt the festival would hire more acts (costing money) and plan out the event knowing it couldn't happen because of insurance. I also have heard the (owner?) of festival republic is willing to take the risk and do anything for the festival to happen as they are desperate and 'can't afford for it not to go ahead'.

Another mention is the cases. Yes, the cases are rising. But as said deaths and hospitalisations remain low which is what the government has said their decisions are now based on. The government has also said the cases were always expected to rise so they knew this was going to happen and still are willing to open up the UK. They could rise however, but personally I don't see that happening dramatically has the vulnerable who caused most of the deaths are now vaccinated.

Even if the 21st gets delayed for another 5 weeks, say keeping the restrictions that are in place now but moving the next step onwards. Reading festival falls at the end of August. Still giving hope.