This was done to prevent buyer and sellers arranging "viewing" visits, then making a private sale off eBay and evade paying eBay's fees. It also left the buyer with no buyer protection.
eBay now scans every message messages between its millions of users to detect attempts to provide contact details. Being caught means at least a warning, and possibly account suspension.
The agreed cancellation procedure if the buyer isn't happy with the item is not always as friendly as it sounds. Irate sellers may instead cancel for non-payment, leaving the buuyer with an unpaid item strike on their account. (eBay would probably remove this on appeal.)