Why are blockbusters now only released in 3D for the first week/weekend?

While I work for a US theater, I honestly want to say this is simply your local theater and/or theater chain doing this, and not necessarily the studios themselves, however I have no real basis for this other than my own observations.

Here in the US, both the studio and theater work out agreements as to how many prints/copies the theater will buy, and how many times the movie will be shown each week, including the earliest and latest start times for the prints they pick up. These can sometimes be renegotiated, but are often set in stone for at least the next week. After that, negotiations take place based on how the film performed in the previous week, and what it's up against that week. They may expand, they may scale it back, they may decide to stop showing it early. Aside from how many prints they buy and how many times they show it, they also negotiate the studio's cut of the profits from ticket sales. This usually scales back each week the movie is in theaters.

Could be the studio is pushing your theater chain to exhibit only 3D showings in the first week, or it could be your theater is deciding to exhibit movies in 3D for the first week before exhibiting 2D. If the theater you go to has very few screens they may be going for the higher upsell in the first week, then scaling back to cheaper screenings in 2D in the following weeks.

The theater I work for here in the Midwest has both 3D and 2D showings, and we actually tend to prioritize 2D showings on our largest screen. Overall we have more than 15 screens total for the location I'm employed with. We have early showings of Guardians of the Galaxy this Thursday, and all early showings on our largest screen are in 2D, though we will be offering the 3D version for patrons who want it.

In the end, the only thing you can do would be to contact your specific theater and ask why. Unfortunately there's likely very little you can do about it, as I can't imagine they'd continue to do it were it not seen as profitable to them, or if the studios are telling them they have to exhibit movies in 3D (when applicable) in the first week if they want to get prints.

I spoke with my local theatre manager and it is indeed a requirement from the studios/distributors that certain movies must be shown in 3D only, for the first week, to capitalize on the extra revenue. It was also mentioned that this summer has had more 3D movies than past summer so it's been a noticeable trend and has been met with many complaints.