Why resolution of Goggle Earth images vary place to place?

The imagery used in google Earth and google Maps comes from a variety of sources ranging from satellite & aerial imagery to imagery taken from air balloons. Some of the imagery is given to google by city or state governments. The age of the imagery varies greatly (from several months to 5 years) and the older imagery will have a lower resolution due to technological improvements.

This imagery comes from a variety of vendors each from cameras with different resolutions and quality. google performs extensive post-processing to stitch and blend the imagery together. It's hard to fathom what a massive project updating the google imagery really is and the costs involved.

Short answer: there is no one source of imagery covering the globe at all levels (national, region, local, etc.).