It’s because they don't want the clinicians and angry shareholders to be swarming their booth asking tough questions around regulatory status in the UK.
With all the things that are lined up to kick off on approval , NWBO will get plenty of attention through PRs and news media. It will still be talked about at ASCO by others.
NWBO clearly does not believe they will necessarily have approval by ASCO so they are not going to go there and have to repeat the same crap everyone has heard year after year, only to have to revamp their messaging a couple of weeks later.
There are plenty of other and better chances for NWBO to get the word out on approval since we will be past the science project phase and into the business phase of NWBO and that means reaching different audiences that we have neglected for most of the last decade.