Because they can take a lot of money from people who buy day one at full price. My advice: if you really really want a pack and to play it immediately, then buy on release. If you can wait, then do so. EA knows they have a large crowd of day one buyers who are willing to pay the full price, and they know they can take in the rest by putting it half price fairly early on. I guess it's a question of how much you want it on that particular day. Me, I've bought certain packs like Seasons and JA on release and don't regret it because I had fun. More recently, I waited for sales for Strangerville and Island Living because I didn't feel I'd get the most value out of them immediately.
The only thing I don't get is why stuff packs don't go 50% off more often, because I would never have bought Movie Hangout if it hadn't. SPs generally don't interest me at £9.99, but £4.99 can sway it.
