I'm not positive about the premise here. There are certainly cases of runners having cyclist like VO2 levels.
Also, cyclists are way more testing conscious. The fact of the matter is elite cycling is a heavily technologically based sport, while elite marathoning is not.
Lance Armstrong was getting wind tunnel tested back in the day. Meanwhile, the Kenyans -the dominant marathoners- run on tracks like this:
Here's their gym,
All the above is from a BBC special on David Rudisha, the world record holder in the 800 meters.
(By the way, this is not to disparage the Kenyans. There is a lot to be said for their training, and them often purposely avoiding a great deal of technology.)
But let's run with it and say e.g. the top 100 cyclists have a better average VO2 max than the top 100 marathoners.
Well, we have one potential reason covered already- the old business phrase of "you get what you measure." Cyclists focus on the number more, they're in the lab more, thus they get better values. It often gets forgotten VO2 max does not correlate too well with running performance. Many runners don't worry about it much. This is most pronounced at the elite level. This is why Lance, only a year removed from winning the Tour, with that high VO2 max, described running a marathon as the hardest event he's ever done.
With VO2 max, we typically are talking relative measures. That is, we get a VO2 value, then we divide by bodyweight for normalization. Otherwise, it's simple: bigger body => bigger VO2 max (provided everything else is the same). Because bigger body => more muscle => more oxygen flowing.
However, normalization doesn't quite do the trick.
Muscle strength correlates well with cross sectional area. Body mass correlates well with body volume.
- Area is length * distance.
- Volume is length * distance * depth
If we view muscle as a rectangular tub, then as the tub increases its dimensions, volume goes up quicker. For example,
- Area starts at 2 * 2 = 4
- Volume starts at 2 * 2 * 2 = 8
If we increase the length, width and depth by 1,
- Area = 3 * 3 = 9
- Volume = 3 * 3 * 3 = 27
Volume is going up faster than area. The area went up roughly double, where volume more than tripled.
So what happens is as people get bigger, their relative strength goes down, because they are adding more muscle volume than strength. Of course, more muscle isn't always detrimental -sprinters are jacked- but in general a heavier person is carrying around more dead weight.
Furthermore, as we get heavier, it gets harder to have the same body fat percentage. A NFL offensive lineman and wide receiver might work out side by side, but it's going to be impossible for the lineman to have the same body fat percentage.
Thus, as athletes get heavier, such as rowers compared to runners, their relative VO2 max goes down.
Marathoners are smaller than cyclists. For instance, Chris Froome weighs ~150 lbs while the world record marathon holder, Dennis Kimetto, weighs 120 lbs. Lance was a 175 lbs at his fighting weight. You won't find (elite) marathoners that heavy.
...What we've just done is make a strong case marathoners should have higher VO2 maxes than cyclists!
This is where we go back to the intro- cycling's infatuation with tech cannot be untethered from its notorious drug issues. What's one way for a 280 lb guy to still be shredded and have abs? Drugs. (Testosterone.)
What's one way to bump your VO2 max irrespective of training? Drugs. (EPO.)
Who is currently in the midst of a drug scandal? Chris Froome.
Elite runners are no saints, but if we go by history, they have nothing near the level of doping issues cycling has had. The Tour in particular has an insane, albeit entertaining, history.
I've heard Lance say guys used to tie fishing wire (hard to see) into a cork, attach it to a motorcycle in front of them, have the cork in their mouth, and get pulled along.
Or this, "There are photographs of riders holding ether-soaked handkerchiefs to their mouths, or leaving them knotted under the chin so the fumes would deaden the pain in their legs."