Freedom of Information requests are slow, tedious, and as a means of gathering information roughly equal in efficiency to the game Battleships. You fire off a formal request, wait a month for a reply telling you your request was rubbish and off-target, move two squares to the left and fire again, and repeat until you get what you were after, or you've given up, or you've died of old age. Apply this to research data and there's a good chance that even if it's available it may be unintelligible, or in a proprietary format, or undocumented.
I don't want to get into a three-month formal e-mail exchange just so I can see some data for a paper. The only request I want to submit is an HTTP request. I want to be able to look up a paper on the internet, read it for free, and then click a button next to it to download the data that was used (where appropriate) in a standard format so I can have a play with it. It seems like a pretty basic thing to ask.