Why does it take so long to store long exposure shots?

I've been taking night time pictures with slow shutter speeds like 2-3mins, why does it take so long to show up on the lcd display for preview? I have a nikon d3100, is it like this for all cameras?

Because there's a lot of anti-noise processing done by the camera's processor for long exposure shots.

Hondo is correct.

All dSLR cameras show this long writing time when you are shooting long exposures.

There will be some problems shooting long exposures. CMOS sensors get hot after only a few minutes generating a lot of noise in and of itself. Add any increase in ISO settings you may use, will cause additional noise. Page 134 discusses noise reduction

You will need to experiment with your exposures to get the best possible results. Try shooting using RAW and then process them in Lightroom. There's a noise reduction tool on it that works very well.

Nikon cameras have a noise filter function. When it is on, the camera does noise reduction as indicated above. However, it can be turned off, which results in a lot faster cycle time.

Nikon calls this "Long Exposure Noise Reduction", and can usually be turned on or off. Find how to turn it off in your manual, and try it on and off and see if it makes any difference. I usually leave it off on my camera as I have not noticed much difference.