Contrast the need to use flexbox with the ability to just specify bold or italic style, or for that matter use <b> or <i> tags. You basically get what you want every time. Nobody has to argue about whether the italicization is the right slope, and while you can select a particular font weight if you want to, bold is bold and is fine for basically everybody.

Hell, I would argue that centered table contents gave people what they wanted most of the time, and if it didn't you could fix it by adding more tables. Tables composed quite nicely and gave you something if you just laid down a table and some TDs.

Flexbox doesn't scale down the way tables do. I have to have both the container and elements inside that container and think about the ramifications of all of that. It's minimum level of complexity is a higher floor than most of the HTML specification.

That's not to say it's the wrong solution for the problem; I think a case can be made that the problem itself is irreducibly complicated. But that's why I say it feels like using an elephant gun relative to much of the rest of the spec; I have a "read my mind" option for bold and italic.