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.
They didn't just cost money. They were competing to the limited space around the typing area, what meant they were constrained at the border of a circumference that would be entirely filled with mechanisms. In other words, the cost in both money, size, and weight depended on the square of the number of keys.
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.