1. The problem in math is not that the way the "orthodoxy" insists on presenting things in a suboptimal way, but that if the reviewers find a good explanation of your result, they'll recommend that you publish in a lower-ranked venue than your result would ordinarily merit (at least unless you solved a famous problem). So researchers are incentivized to make the presentations of the proofs as opaque as possible. You can see this in conference and workshop talks (which tend to happen pre-publication in math), in many fields speakers always avoid presenting _any_ proofs. Putting better explanations in an appendix, which the referees can read, simply wouldn't help.
2. Being easy-to-understand at first is actively punished, but even being easy-to-understand in the long run is not rewarded. You can always write an explanatory blog post after the publication decision has been made, but you won't put effort into writing one if you don't gain anything from it. This applies even more to the people in the example, who wrote on a typewriter in the 1970s. There were no blogs, and it was much harder to get an appendix through because page limits were physical limits, and the act of writing was much more onerous before the age of computers and LaTeX. There was no point to doing it given that it was actively discouraged.