It's millions of files of plaintext, each multipled hundreds of thousands of times by continual edits. That adds up, even with compression.

You're right in that storage space is relatively cheap, and with the right funding and infrastructure, it could be done, but Wikipedia isn't Youtube. It doesn't have multi-million dollar venture capital funding and hoards of advertisers waiting to give them money. It's mostly a volunteer operation being run by left-wing partisans who have to make every penny count. That's why they run that banner ad begging for money every other month.

Case in point, Imgur doesn't store images forever. It deletes them after a while. Youtube keeps things around indefinitely, but they're basically Google, and they're using that content to drive revenue, so they can afford it.