@ThisisJ 

I would suspect it has to do with memory space on the router. The next logical number to increase to would be 128 then 256. 

You would think that it would be much higher as everyone is getting more devices and the internet bandwidth has incresed that can support more devices.

Edit:

One could add a device such as PFsense to whitelist MAC address as the doc says it is unlimited. However, whitelist of a MAC addresses does have it place, it is not much of a security featue it once was as the MAC address can be spoofed. Look at the iPhone and iPads that have a feature that changes the MAC address. You need to turn that featue off to use whitelist.