The decision of a default encoding surely dates back to Python 1.0 or earlier, which predates not just UTF-8 but even Unicode itself. Python is an old language!
And if the assertion is that Python 2.0 should have made the tumultuous Unicode jump when it released in 2000, I could get behind that (especially in retrospect!), but enthusiasm for both Unicode and UTF-8 was not nearly as high then as it is today, so I don't begrudge them for not jumping at the opportunity.