So many of the native bees were and still are killed off by pesticides. Native bees also are not as efficent in polinating large tracts of single crops. Farmers became dependant of honeybees when they wanted large numbers of bees to polinate a specific crop and then be gone and out of the way. Thus the practice of moving bees from place to place. Now bees are moved North to South and kept active and productive 12 months of the year. Some of us believe that this has contributed to the weekness and suseptability of the modern bee.
There is a movement afoot to preserve and provide habitat for native polinators. It's important to keep the biodiversity alive, when faced with decline in a specific species. It seems like whenever man tries to interviene or control nature, it backfires.
Also, just to piggyback on the previous poster's response, there were many food crops brought over by Europeans that we are dependent on that are not native to North America. Many fruits and grains depend on the European honeybee more so than the other species of bees that are native to the area for pollination.