Wow. I genuinely thought he had the guts to go. Won't make that mistake again.
I don't think that its a question of not having the guts to go. I think its a question of not having any motivation to go.
If you were invited to attend a dinner organised by a group of people who you didn't particularly like and who didn't particularly like you would you attend? What possible motivation would you have for attending?
Previous Presidents have attended in order to demonstrate both to the WHCA and to the world that beneath the surface confrontation they really get on with the 'mainstream' press. For 35 years they have accepted with good humour the gentle mockery of the comedian making the 'roast' as part of the same demonstration. President Trump has absolutely no motivation whatsoever to make such a demonstration, quite the reverse. Therefore he has absolutely no reason to go.