The underlying of the capacity option is the block gas limit. Block gas limit defines how much computation and storage can be used within the single block.
Block gas limit fluctuation can be caused by one of the few factors:
miners deciding to move the block size up or down (each miner can change the block size within allowed range and the majority of miners can collectively move the block gas limit in any direction)
core protocol developers advising miners to change the block gas limit and miners following the advise
core protocol developers introducing a network upgrade changing the block gas limit
When buying or selling a block gas limit option you are taking a view on one or all of the above - you may expect a miner induced network attack, a core developers induced protocol change or a natural agreement to move the block gas limit in any direction due to technological progress or protocol limitations.
Historical block gas limit chart (see etherscan.io for the current values):