Some of the comments on here are so mean spirited and no worse than the 'waity kaity ' shit the media ran at the time.
Maybe she was in no rush? Maybe she wasn't sure she was willing to be in the public eye as Diana 2? Given they met at uni it would have been unusual to have it done and dusted within a few years, especially given his position. Yet there are some who extrapolate from their courtship that she's now some dim wifey who will put up with anything to maintain her position.
And actually do you know what - maybe if I happened to fall in love with a future king I'd wait a few years for a proposal. Doesn't seem like the worst life in the world!