There are a couple reasons why an external HDD, or HDD enclosure might have a limit.
With a Basic HDD using the MBR (master boot record) partition type, it was set up to using 32 bit addressing. 32 bits can address 4.294 billion different sectors x 512 bytes per sector = 2.2 TB of space. If there were 2 2TB drives in an enclosure, it could hold twice that amount. If a HDD were set up using the newer GPT partition type, it theoretically could use up to 32 Disks, or up to 64 TB.
Also there may be a limit of address space in Network Area Storag enclosures because of the OS of the unit, such as Linux.
So it just depends on whether it is using 32bit or 64bit addressing, how many physical drives an enclosure can hold, and the OS used.