Why can't you rotate Map view in Google Maps or Bing Maps?

It's actually for the same reason that you can only zoom in or out to specific levels that have been pre-defined, instead of a smooth, continuous zoom to any scale you like.

The reason is that all these maps are made from square image "tiles" which fit seamlessly together when they have all been downloaded. If you are on a slow connection, you can actually see these image tiles arriving and being displayed one by one, and if one is slower to download than its neighbours, you see a square hole in the map until it arrives.

All these images which make up the map are drawn on the provider's servers, not in your browser, and they include all the map labels and symbols too. These are all drawn with the assumption that north will be "up". So the map can't be rotated, because the labels and symbols would end up unreadable, upside down.