We know Mr. Weasley had the magic expandible tent as the original owner wizard Perkins didn't feel well enough to use it for camping any more. So when they had the Delacour family, couldn't they have shifted the younger members to the tent instead of cramming them into shared bedrooms?
I'm not aware of canon answer.
It's possible that the tent wasn't as secure as the Burrow itself.
The wedding happened in the atmosphere of super-heightened security (just prior to the fall of the Ministry, post-Battle of Seven Potters, with Harry there).
Burrow had a lot of security wards put on it specifically to secure it... and the tent may not have been as secured as the house.
Perhaps Hermione had already packed the tent in her bag
HERMIONE: I've had all the essentials packed for days.
If Hermione had been packing all the essentials for days before the wedding had even started, then it's possible that she had already packed the tent away in her beaded bag, making it unavailable for use at the wedding.
Note that this is also a likely out-of-universe reason as well; the trio needed the tent for their wanderings, so JKR had the wedding use something different.