Its not only Android devices that have to wait for updates, but pretty much every other device including BlackBerry and Windows, except iPhones.
One of the reasons is because each carrier must subject each update to a testing process before releasing it, if they ever do! basically when you get a phone from a carrier like O2 they have customised the phone, adding their own branding and bloatware to it. The carrier exercises control over your phone even after you buy it, ie by locking it so it cant be used on another cellular network. They are the ones in control of the phone and the software installed on it, and so they are the ones in controlling of approving and rolling out updates.
When a new version of Android, or BB10 or Windows is released it is the carrier's job to test all the different updates for all the smartphones they sell and this may take many months to test effectively. They might even decline to do the work and never release the update!
Apple on the other hand used the popularity of their first iPhone to upset this process.Apple informed carriers that they were in charge of the phone. Carriers arent allowed to install their own software or brand it with their logos. People want the iPhone, so carriers want to offer it on their network. Apple uses this as leverage to exert their power over carriers, and carriers can't hold back iPhone updates for the same reason they can't ship iPhones with bloatware.
If you remember back a few months with iOS 6.1, a number of problems occured for Vodafone UK and 3 Austria after this update and a couple of updates thereafter. The carriers solution for customers was not to update to iOS 6.1 as they couldnt block the update as it wasn't in their control.
So in summary...Android don't get updates promptly because it involves work by O2 to test the updates, and iPhones dont have an update delay because Apple rules everything and can basically do what they want.
