It’s the scoring in decathlon that keeps it obscure.
If I tell a non-track fan that Kichoge ran 26.2 miles in 2:01, they know what that means. They can immediately conceptualize the enormity of that accomplishment. With a little arithmetic they can break it down to units they might have personally run - “oh my gosh, that means he ran one lap of the track in 69 seconds... for 26 miles! I can’t event do that once!”
If I tell them someone set a world record in decathlon, scoring over 9100 points... no one knows what that means:
“How do you know how many points to give?”
“Well, there’s a book...”
“And how many points do you get for running the 100m?”
“Depends how fast you run.”
“Well how many points would Usain Bolt get?”
“I don’t know, but this guy didn’t run nearly that fast.”
“Well how many points did he get for his 100mm?”
“I don’t know, I’d have to look it up...”
To this, add the fact that non-fans are always going to be most impressed by records that demonstrate the extreme limits of human performance. A guy who set a record for the best combined score in 10 events, but couldn’t even get an Olympic medal, much less a world record, in any of them, is not going to be as impressive as the guy who ran the fastest marathon ever, period.
Finally, the 10 events are abitrary. Take out the 1500 and replace it with a 10k, and you have a whole different set of champions. You could say the marathon distance is arbitrary, but change it to 25 miles or 20 and you don’t fundamentally alter the challenge - Kipchoge is still your champion.