muscle efficiency.
Cycling specific does not translate to running efficient muscles. And visa versa. And same goes for adding swimming.
I used to run and cycle and enjoy both. I quit running because terrible bio-mechanics were leading to injuries. When I tried swimming, I was unable to get half a lap without a 5 min break. Just not efficient.

Cycling is the best. There is no way I can comfortably take a coffee break in the middle of a run. Tried it once and just didn't work. Cycling though!! I can work in many donut & coffee breaks!