Because I rarely find nukes with invulnerability globes outside of 100. Finding those two things together outside of 100 is scarce. Not to mention that I typically save few nukes I find in a standard run to attempt full win if I am so inclined.
If I know for sure I can nuke a floor while invincible, I might. Why am I hesitant? Loot. Barring uniques, but still. Loot. Heals. Ammo. Possibly good armor, if I didn't have it already. If I put a nuke down, it's because I cleared the floor already, found the stairs, and am standing one tile diagonally away from the stairs. The nuke is just for giggles.
The only exception I'll make for nuking is the sweet Trigun.
Because I have to be invulnerable when I Angel Arm.