The Rosh and the Rambam argue about whether Kiddushin is a mitzvah in its own right or if it's a preparation for the various mitzvos that can only be done when married, such as having children. The Rosh (Kesubos 1:12) holds of the latter view; the Rambam (Ishus 1:1-2) holds of the former view.
Generally speaking, Ashkenazim follow the Rosh, who himself was Ashkenazi, while the Sefardim follow the Rambam, who was also Sefardi. (This is a bit of an oversimplification, but that's the general idea: Ashkenazim follow Ashkenazi poskim, and likewise for Sefardim.) Thus, it's understandable why Ashkenazim don't make a Shehechiyanu under the chuppah, as we hold that it's not a mitzvah in its own right. But why do Sefardim only make a brachah because of the Tallis (see this article, at the end of the "Chuppah" section)? If they hold like the Rambam, they should be able to make a Shehechiyanu without having a Tallis!