My only guess is that they feel that they are in a new era now, where before potential death was considered an accepted hazard, but now they want to establish that it isn't, shouldn't be, and won't be.
That's the only reason I can think of, aside from the somewhat morbid admission that if they did it for every driver that died during F1 activities previously, there would be very few numbers left.
Oh, wait. Didn't they used to have numbers assigned to/selected by the manufacturers, and just in the last year or so they switched to numbers being assigned to drivers that stay with them their whole career? Perhaps that's another reason.