For one, they know that staying upbeat is more infectious than constantly fretting about "concerns".
I'd wager that they also know what we all know - the polls are rigged and he's doing much better than most realize - I'd wager that if they leaked a lot of their internals they'd have to worry about Trump supporters getting complacent.
If IBD weighted D+2 instead of D+8 which is more in line with the electorate, Trump would be up 8 points right now!