I didn't say people
wouldn't do it. Some people actually did! the whole Maestro team, for most of Q2 2017, for instance.
Part of my point though is that converting coffeescript to typescript was very boring, mostly mechanical work (which is more obvious in hindsight, since we actually automated most of it). Sure, you could make the code better while you are there, but that actually was likely to add bugs, which could significantly delay such conversions, or lead you to chasing around the codebase to figure out how to get typechecking to work with your attempt at better types. What you are describing - going through an old codebase and figuring out how to fix it - sounds a lot more interesting! though definitely there are plenty of people who wouldn't want that job either.
But really the biggest blocker to executing on the manual conversion strategy was management. They already had other commitments for their teams, and spending a few thousand hours converting coffeescript to typescript wasn't as high of a priority for most of them, than the things their teams were actually working on. This partly comes down to our incentive structure - doing work for other teams, especially in a migration like this, can often feel like a chore, and is often not really rewarded with anything. But prioritization for a large project like this also takes a lot of input from upper management, and they didn't want to sacrifice other priorities to keep the original manual conversion plan on track.