Here's the answer from Google themselves:
"Google doesn't return all backlinks in response to a link: command. In the ancient days, it was because there was a finite amount of storage space on the machines that served link: requests. So we only kept the backlinks for the top N pages. Later as we moved to a different indexing system, we kept backlinks for the top M% of pages. This was helpful for important pages, but it meant that Mom and Pop sites with lower PageRank wouldn't have as good a chance to see their backlinks.
At SES London, DaveN had a suggestion. He said: why don't you give all pages an equal chance of seeing backlinks? That's good for users, who will have a greater chance of seeing backlinks for a given page, and it's especially good for smaller websites--they'd have a chance to see backlinks. It seemed like a good idea, so we implemented it. In fact, in order to give each page a better chance of seeing backlinks (instead of just the top M% of pages), we doubled the amount of backlinks that Google exports to the outside world. So users now have access to twice as much link: data as before; it's just not all the top PageRank pages.
... and ...
Given the large amount of data involved, and the fact that remarkably few people use the link: command, we don't show all backlinks externally, but we can access them internally."
Hope that helps
