Because nothing says patriotism like saying you're more popular than one of our nation's greatest tragedies.
Seriously though, when did Republicans suddenly become the unpatriotic party? Maybe they always were and I just never noticed it. Not to say Democrats weren't patriotic, but that was one of the GOPs things. No more can they claim they're a party of patriots.
Yeah, this is pretty ghastly, but it's standard for Trump at this point. It's all about him, and it doesn't matter how everything else falls into context.