Hmm, not sure...really?
Well, nothing is 100% of course. However, why should Doctor Strange 1 and Black Panther 1 get hot if not for the fact that they are # 1 issues? We're seeing the same thing with Inhumans 1. None of these are first appearances (not even close) and yet they are in high demand right now.
Nobody seems to want Black Goliath # 1. However, if he's put in a movie then BOING! BG 1 isn't a first appearance either. However, it's a # 1 and would blow up if a movie came into play. When people pony up BIG bucks for 2nd appearances (think Rocket Raccoon and Black Adam) nothing surprises me anymore.
Sometimes (most times) it has to do with finances. I don't have hundreds or thousands to spend on high-end keys, so why not a nice copy of an affordable #1 or 2nd appearance? The IH 271 Rocket Raccoon is awful looking in my opinion, so I'd rather have a Rocket Raccoon #1 that looks more like the movie character (speculation?) than the IH 271 version. I can also get a nice copy of Doctor Strange #1 or Black Panther #1 waaaaaaaaaaaaay cheaper than an ST 110 or Fantastic Four 52.
BIG bucks are what people are ponying up for those GA/SA issues. If you're a big baller and can get those, then kudos. Not everyone is on the same playing field when it comes to money.