You have asked a question scientist have been researching for years. Common theories are:
1.) Pesticides. These are chemicals designed to kill insects...bees and butterflies are insects.
2.) Lawns. Grass doesn't produce food for pollinators. A natural meadow or vacant lot, in contrast contains many flowers that bloom at different times. Douse your lawn in chemical fertilizers and things get much worse.
3.) Colony Collapse Disorder is affecting bees.

let's get a little more specific :-)

** Insecticides are intended to control insects (the term "pesticide" covers a much broader spectrum) and some insecticides - neonicotinoids in particular - are having long term effects on bees. And may be (but are not yet proven to be) a factor in Colony Collapse disorder.

** Loss of habitat. Lawns are only one indicator of this but new construction and the clearing of natural areas for development results in the same loss of habitat that any native species must endure. Some are much better at it than others.

** Lack of biodiversity. This kind of goes along with the loss of habitat as the two are related. More developed and more cultivated land typically signifies a reduction in biodiversity both with regards to food sources and the proliferation of beneficial and harmonious species.

** Climate change. Radical changes in weather patterns/colder winters/hotter and drier summers/ more severe storms, etc. affect all aspects of the environment, including pollinating insects.