Anecdotally, 2 (almost 3 but she REALLY hated hills) of the cyclists on my university team were ex-rowers. Cycling is way more accessible than rowing, from what I can tell. You can do it solo or with just a couple friends and you don't need water.
Asking why rowers make good cyclists is like asking why runners make good cyclists. These are all aerobic activities that use your legs. Not much to be said there.