Because without fixed deltaTime, if the framerate is too low, you can't jump in Quake. With the "new algorithm", in the 3fps example, you still have to luck out and get a tick at the right time.
And with a constant delta, if you choose it right, it can be easier to get collision detection right. You can possibly get away with checking if two things are colliding with each other right this tick instead of checking if they might have run through each other between ticks. With variable delta, if you want to get it right, you might have to check where things where half a frame ago and such, and at that point it might be easier to just check more often instead.
And I think you should probably ask the same question about variable deltaTime. Assuming that coupling rendering and game logic is not some best practice that you should default to, why would you want variable deltaTime? (There may or may not be some good answers to that, and maybe it totally depends on the game and so on.)
