The Sochotchover (Ne'os Deshe) has an interesting definition/hagdarah of what constitutes a seudas mitzvah. The Midrash (Shir haShirim) writes that there is an obligation to make a seudas mitzvah upon the completion of the Torah. The gemara (Shabbos 119) tells us that Abayei would make a siyum = seudas mitzvah upon finishing a masechta. We see that the idea of seudas mitzvah is related to the completion of a mitzvah. The reason for a seudah to celebrate a wedding is because it marks the completion of the state of "zacher u'nekeivah bera'am," the unification of husband and wife that is the whole of "personhood." Shabbos obligates seudah because it is the completion of the cycle of creation.
So why is there no seudas mitzvah for
Chanukah to celebrate the completion of the rededication of the Mikdash?
The Sochotchover answers that the completion of the Mikdash is not
an end in itself, but is really a new beginning. It is the mechayeiv
of avodah, not the completion of an avodah.
(OK, so you will ask me why we can't
say the same about a wedding. Point granted.)