The Yerushalmi derives from Lavan's instruction to Ya'akov Avinu, "Malei shavua zos," to wait until sheva brachos with Leah are over before he marries Rachel, that one cannot mix two simchos together -- ain m'arvin simcha b'simcha. Even though the Yerushalmi holds that one cannot learn halachos from events pre-mattan Torah, the pasuk establishes that one cannot emotionally dance at two weddings, i.e. it gives us an insight into the facts on the ground from which the halacha then follows (for more on this issue, see this post at Havolim).
Based on this din, the Rambam paskens that one cannot get married on Yom Tov since ain m'arvin simcha b'simcha. Yet, the Rambam also paskens that one is permitted to marry multiple wives at one time, so long as the sheva bracha of each is celebrated independently. In other words, after one big wedding ceremony the chosson will in week 1 celebrate sheva brachos with wife#1, in week 2 he will celebrate sheva brachos with wife#2, and so on. Why then can one not get married on Yom Tov and simply celebrate sheva brachos a week later after Yom Tov? Just like one can get marry two wives simultaneously and celebrate the sheva brachos of wife #2 a week after the fact, why can't a person get married on Y"T and celebrate sheva brachos a week later?
The Brisker Rav answers that there are two dinim in sheva brachos: 1) The person getting married has a special status as chosson; 2) A din in the simcha being celebrated not being interrupted.
A person who marries multiple wives has a chalos shem chosson multiple times over. The only concern is that the simcha of each wedding not interfere with any other wedding. That can be accomplished by breaking up the sheva brachos into multiple weeks.
However, if a person gets married on Yom Tov, the celebration of Yom Tov prevents the shem chosson from being chal. It's not enough to put the sheva brachos after Yom Tov, because at the critical moment of chassuna the shem chosson will not fully take effect.
I seem to recall once hearing (and I haven't seen it inside) this logic to explain why if a sheva brachos meal on the last day spills over into nightfall, sheva brachos should no longer be recited. Even though when it comes to additions into bentching, e.g. ya'aleh v'yavo, al hanissim, we usually look at when the mean started, sheva brachos is an exception. Since it is not the meal which obligates the recitation of sheva brachos, but rather it is the presence of the chosson and kallah, once the seventh day ends and the chosson no longer has that special status and chalos shem, the brachos cannot be said.