Because up until the time he disappeared there pretty much wasn't anywhere to spend a single Satoshi? Maybe some socks but even they might have come long after. Even if there had been a healthy commerce scene I assume he would've been wary of spending on digital or real products for paranoia reasons.
Imagine you have a million Bitcoins and the price goes to 1000$ and more. Wouldn't you or any normal person sell at least some part of it? Don't tell me he doesn't care about money or he wants to wait for a higher price. This makes no sense.
it was said that his amount was only 200k and not 1M, even so selling 1k at the last peak will not be seen easily, so it may be possible for him to sell something without none notice it
maybe he is betting on his own prediction, about huge volume or no volume, and wait for a very high value
At the time he disappeared, Bitcoin was worth about 0.30$. Short after it broke the 1$ hurdle and Satoshi was a (theoretical) millionaire already. Now imagine Satoshi when it went 10$, 100$ and then it even goes over 1000$. One needs balls of steel to resist to sell some at this point, I think.
If this hodl theory is real, I would think that this Mr. Nakamoto is kind of a psycho. Why disappearing then and give control over your code/website etc to a stranger? If money would be your motivation, wouldn't you act different? I think, this theory only works, when thinking of Satoshi being a programmer with an inferiority complex. He has this genius moment where he invents Bitcoin and as he knows about the importance to the world.
So far so good. Up to this point he still looks like a normal genious and pretty idealistic, but the fact, that he instamines a big part and keeps it secret and untouched for years, makes him look pretty much like a freak. I would then assume he was going for "power" instead of "money" and he was waiting to "rule the world when Bitcoins is worth 10.000 or 100.000 or more". This sounds pretty much like a freak to me.