Typewriter keys cost money, and dropping the 1 allowed them to drop a key without significantly affecting the use of it. As far as I can tell, that's effectively the entire rationale.

This wasn't meaningfully the case prior; the printing press would've just needed more copies of 'l' if they'd dropped the 1s, and letters weren't as significant a portion of the cost of the machine, anyway. And afterwards came computers, which need to distinguish between the characters even if they're displayed the same way.