Usually, your carrier will have some role in if the updates get pushed.
As an example in Canada, there are two main groups of carriers, AWS, and Other (I don't know the name). AWS carriers service a miniscule minority of Canadians, but the handsets they sell require different radios and therefore different software to run. So companies push out the "Other" carriers' update first because it's more efficient.
Some carriers are horrific for holding up the process, I've read this of Verizon before. Basically, your carrier may not have any direct control over the update, but depending on their standing and the sort of changes that need to be made to accomodate their networks, the update will be delayed if it's not a priority (usually in...