This is one of those problems that sounds reasonable if you think about it on an individual level..."I have flown XX times it's not that big a deal to show me my entire travel history" And you're right, it's probably not that big a deal to show you your entire travel history...
The problems arise when you need to have everyone's entire travel history. I can't imagine the size of the server you would need to have all that data accessible online. Plus out of the millions of flyers, how many people would ever actually look at it? (If you take out FT'ers)