First, Nickelodeon knows that iParty with Victorious will get tons of views because it is a crossover and they aired lots of promos beforehand. With all of those viewers watching, it would be a perfect time to air a promo for iDate Sam and Freddie.
Second, in Dan's latest blog post, he said that the episode will air in a few weeks. In my opinion, a few weeks in anywhere from 2-4. With iPWV just a week away, I think it would the right time to start airing sneak peeks.
Lastly, I'm pretty sure Nick is aware that "ship" episodes get lots of views with iSaved Your Life getting about 13 million views and iOMG getting 7 million views. Therefore, they will probably want to let iCarly watchers to know about the episode as soon as possible to build up a viewership for the day it airs.