Citra hops are both extremely popular and a proprietary trademarked variety, so while more acres will go into production in the future the demand clearly exceeds the current supply. It takes about two years from planting to good commercial production off a row of hop bines, so answering the demand takes time. This is why major brewers and hop suppliers have contracts for specific hops years in advance, promising the growers that they will take those varieties.
A portion of available hops are routinely packaged in small units, just for the homebrew market. This means that even if it is hard to find a mail order source for an exotic hop you may actually have it better than some of the new commercial breweries trying to make trendy exciting beers. Bigger operations might not be able to get larger packages of a particular hop at any price.
This issue comes up with the most desired private sector hop strains. Here is an interesting article from a grower's perspective about another licensed hop variety, Simcoe, which had a shortage situation a few years ago.
Just for reference, hops always used to be developed by universities and then be growable by any farmer without a licensing fee. Some new public sector hops are just coming out of the Pacific Northwest: http://thebeerdiaries.tv/4-new-public-hop-varieties/ Within a few years we may be able to order some or all of them. Hopefully those four will not be the last "free to grow" species developed, due to university funding issues.