That's a bit like asking why I work out instead of snapping my fingers and turning into a Greek demigod. One is clearly preferable on its own merits — but it's just not going to happen any time soon.
Source maps are relatively easy: There's a public standard, a couple of browser makers have agreed to it, and it doesn't need universal acceptance since it's only for development. Basically, it doesn't require a lot of coordination.
A universal bytecode is almost the exact opposite — easy to describe in the abstract, but a coordination nightmare. I've never heard an actionable plan for making it happen. Just getting the community to agree on a standard bytecode would be a Herculean task†, and then you have to get Apple, Google, Mozilla and Microsoft to all say, "Yes, I'm willing to chain my VM to this bytecode at significant engineering expense."
Basically, feel free to work on this if you want, but you can't expect everybody else to hold their breath until it happens.
† You might think that "Herculean task" is an exaggeration, but just look at how much discussion and compromise was needed to get agreement on stabby function syntax for Harmony. And still people kvetched even once there was "agreement"!