I think the Counter Strike explanation is a perfect starting point for this. I played Counter Strike from 2001 to 2006 and the only reason I stuck with it the extra couple years was my ownership of a laptop which was not in any ways meant for gaming. When I quit 1.6 I was pretty damn good at it, I could shoot to the top of the scores in most servers so it wasn't easy to make the switch. Fundamentally CS:S is the same game but there are little things which make all the difference like bullet patterns, movement (I still get pissed when I touch a wall and freeze for that split second), sounds, and a ton of other things which I didn't even consciously notice, it was all programmed into muscle memory. However, the out of control hacking in 1.6, the number of wide ranging ban lists I had managed to get onto and a recent loss of clan affiliation got me to make the switch.
Now I play CS:S, not competitively or really that well because I've grown out of stat whoring, and it's just as much fun as 1.6 ever was. The moral of the story is that when you play a game at a high level of skill you become unconsciously trained in it's every detail and it isn't easy to give that up and learn a new game. I was heavily pushed to give it up so I jumped that hurdle but there are thousands of people who ***** and moan about CS:S and how it isn't as great as 1.6 when what they really mean is "It isn't EXACTLY 1.6".
Starcraft has a rabid fan base with a large (I assume, I don't know how many people play SC on Battle.net) group of people who are versed in its every intricacy. Many of these people will not want to:
A. Give up these hard won skills to learn a new game otherwise they wouldn't still be playing Starcraft.
B. Sully their skills by playing with the enormous influx of "noobs" that a game release brings (and possibly lose to them).
C. Stick with it until they get up to the same level of skill because it took years and they aren't going to want to spend more years getting used to a new game.
No matter what Blizzard does SC2 is not going to be EXACTLY like SC because it isn't possible to program all of the quirks that a player becomes used to without realizing it. Even though it's fundamentally the same game the differences will be enormous to someone who is intimately familiar with the original.