Whilst Zenimax didn't mention exactly why they disabled guild banking, it was disabled shortly after a Reddit post pointed out an exploit that allowed players to potentially get infinite gear and currency.
Zenimax has since banned thousands of cheaters according to community forum members, with most of the bans being related to in-game exploits and cheats. It sounds like it's been a rocky start for Zenimax and Elder Scrolls Online, but things will most likely get smoothed out in the future.
If you're a little disappointed that the console release of the Elder Scrolls Online has been pushed back past the PC release, hopefully this news should reassure you a little. MMOs are known for having troubles at their launch, and this is usually because the games are filled with thousands of hours of content, and it's hard for most bugs to found by in-house game testing, and even through various beta tests, some bugs still remain hidden until the floodgates open up and the game releases fully.
The Elder Scrolls Online was released on PC on April 4th, and will be made available on both PlayStation 4 and the Xbox One some time in June. Hopefully by that time all of the game breaking issues will have been resolved.