Well answered by Neal there.
You'll also notice the other historical gaps on the same band too such as between channels 3 and 4, 7 & 8, 11 & 12, also 15 & 16 as well as 19 & 20.
They were originally designated for radio control and over time were known as the 'Alpha' channels with CB'ers utilising them for their own use. (Some called them the 'secret' or 'hidden' channels but they were never intended for CB use.) They're spaced apart more than CB channels due to the simpler and therefore lighter radio receivers used for radio control avoiding the 455kHz IF or intermediate frequency so they didn't interfere with each other.
Imagine spending tens or hundreds of hours patiently and lovingly hand building an RC aircraft only to see it suddenly as well as unexpectedly fall from the sky whilst flying it so crashing to pieces as it hits the ground!! Not helped as a car rolls past with a massive antenna atop it and a CB 'breaker' screaming out "10-4 good buddy".
However, much like how people could've caught more fish with a 'better rod', driven faster if the tyres weren't worn or beat that high score with a 'better joystick', lack of aerodynamic or radio knowledge could easily be blamed on someone else.
It's stuck in history now as you would've otherwise rendered original CB's obsolete as well as the stockpile of preprogrammed PLL (phase locked loop) chips the CB manufacturers had at that time.
All interesting stuff though with the history and evolution of CB radio still there for all to see.