As you say, I think one reason may be that Impossible film is getting better. Another reason may be that the folks over at the Film Photography Project are really pushing Polaroid right now (especially on their podcast).

There is a widely respected author/blogger in the woodworking world (focusing on hand tools in particular) named Christopher Schwarz. There's a joke that goes around that as soon as he finds an old tool he likes, ebay prices skyrocket because all of a sudden everyone wants one. Perhaps a similar effect is happening to Polaroid with FPP's encouragement?