In case it's not yet clear...
Suppose you built a single-row eight-cylinder radial (a four-stroke-cycle radial, which I assume the well-known radials were). All the cylinders connect to the same crank-- right? So the #1 piston starts its downstroke, then the #2, then the #3... but as you go around the circle you'd like power strokes to alternate with intake strokes. So if #1 fires, then #2 does its intake, #3 fires, and so on around to #7 fires, #8 intakes... and then #1 is scheduled for an intake. So now what?
But with an odd number of cylinders that problem doesn't arise.
A typo in the previous post: the R-1830 has two rows of seven cylinders, of course.