To prevent vandalism and spam. You can gain edit access by making a payment (in bitcoins, of course).
The payment required is (at least) 0.001 BTC, which is actually no longer trivially small in my opinion.
BitcoinPayment is a MediaWiki extension to limit spam, by requiring a small one-time bitcoin payment before a user is allowed to edit or create pages.
While the payment is deliberately trivially small for a normal user, it presents a barrier to a mass-spammer.
Funds sent to this wiki will be used to keep the wiki online (cost of domain name and hosting), and remaining funds will be donated to the Bitcoin Foundation.