It's simple... people don't always think things through, and games have to launch with more and more content to be competitive in the market.
So let's not compare SWTOR to WoW... let's make an analogy involving Apple and Company X.
Apple has been around for a while, and has a very good, very small, and very focused product line.
Let's say some years after Apple's "launch" or IPO, Company X wants to be "the next Apple". Well, it won't cut it just to beat what Apple had going for it all those years ago, they have to match what Apple has today.
See, you aren't competing with WoW at launch, you're competing with WoW now. That's why people expect so much from a new MMO at launch, it's because they've come to expect it.
Side note: I wonder if this raising-the-bar trend for MMO's will eventually block out any new entrants to the market... OMG ANTITRUST LAWSUIT NAOW.