My son is also starting to enjoy Duplo, he's mostly using my old sets from about 1986 and they stick together really well. For christmas some relatives bought him a new duplo set and it is noticeably less sticky. I suspect they're trying to make it easier for very young children to casually play with, but it comes at the expense of being able to make the really tall structures that kids inevitably want to build.
My hunch is that if Duplo was built out of ABS like the standard Lego bricks, they would be a lot harder, a lot heavier, and an order of magnitude more expensive. The softness of the cheaper plastic, as mentioned above, is a lot more friendly to toddlers as well.
