A more interesting story is why are the older services using mainly odd port numbers (ie. 21 = FTP, 23 = telnet, why was port 22 free at that time?). It turns out because the protocol which preceded the invention of TCP, called NCP, used even-odd pairs of port numbers, with even for "outgoing" data and odd for "incoming". So the well-known port numbers for incoming services were all odd.
Sources:
https://en.wikipedia.org/wiki/Network_Control_Program
http://www.pcvr.nl/tcpip/introduc.htm