Ah, that makes a lot of sense. I'm a little too young to have experience with typewriters. I assume most European typewriter layouts were even more spatially constrained than the US one (due to the presence of special characters not found in the standard Latin Alphabet), so there wasn't much room to allow two quotation marks. And the fixed-width typewriter "font" didn't look properly typographic anyway.

If the straight quote hadn't been invented, the influence in programming languages would have been interesting. Then presumably there would be no common quotation mark present on all keyboards. So perhaps programming languages would have used various kinds of brackets only. Straight quotes aren't great anyway for most things, as they can't really be nested unambiguously.