First of all, it wouldn't be at Moscone. Apple only rents that facility for WWDC.
The media events are just one day, with a 1.5-2 hour stage presentation. However, they require 2-3 days of setup and the day after to teardown. They couldn't just book the Yerba Buena theatre for one day.
When they do a Tuesday event at Yerba Buena, there's activity the Friday before: event contractors (e.g., A/V), wireless access points, etc. They don't come in Monday morning. My guess is they set up Friday and Saturday, take Sunday off, then do a dry run sometime on Monday.
If Apple uses the town hall facility on their campus, all that activity is cloaked from the general public. As someone else mentioned, it's a smaller venue so fewer invitations would go out; the B-Listers would be shut out.
They want a decent sized venue for the iPhone announcement. After all, the iPhone is their cash cow, they want maximum coverage.