Because the west coast has winds blowing off the water (because of the earths rotation and the prevailing westerlies). The waters along the west coast are much cooler than the east coast during the summer so if the winds blew off the east coast, a lot of the eastern US would be pretty tropical. NYC wouldnt even see snow to give you an idea. I would guess that day time highs in the WINTER in virginia beach would be in the upper 60s with nights in the low to mid 40s if winds blew off the east coast and summers would be cooler with daytime highs getting in the low to mid 80s and lows in the low 70s. Much milder during the winter and in the summer which means warmer winter temps and slightly cooler summer temps also. NYC would probably have a January high temperature in the upper 50s and a low temperature in the upper 30s to low 40s and a July high temperature in the mid 80s with a low temperature in the low 70s.
I just came up with those temperatures in my head, but the point is that the water does moderate temperatures a lot and even though the waters in the west coast are much cooler than the east coast (ex is san diego with a water temp in the low 60s and NYC with a water temperature in the upper 70s this time of the year) but since the wind blows off the water on the west coast and the mountains block inland winds, the west coast is milder than the east coast.
