ocean temps, weather and altitudeI second the comment about the Atlantic. The Gulf of Mexico is also a factor. The east coast is on the lee side of the continent, and given the west to east prevailing winds you would expect that all of the air reaching the coast would be dry and wrung out. However, Florida and the Gulf are far enough south that they in the trade winds, with prevailing east-west flow. Moisture from the Atlantic and the Gulf of Mexico gets fed up into the vortex of the lower Mississippi Valley, where it gets caught up in the middle latitude westerlies and drawn over the eastern seaboard.
Most east coast weather is the result of the interplay between moist air from the Gulf of Mexico and cold dry air from central and northern Canada.
If a low pressure system develops along the Mississippi or Ohio River valleys, the near-surface counterclockwise circulation around the low pulls additional moist air off the Atlantic Ocean. The Gulf Stream, which runs right offshore along the coast until Cape Hatteras, and is only 200 or so miles off the new England Coast, has surface temps approaching 80 F. This tends to produce above-freezing temps at mid-altitudes, even when there is cold air at the surface. If this air is drawn well inland, at best you get sleet and freezing rain in the early stages of a storm, and pure rain as the system pushes any surface cold air away.
If the low tracks just offshore, however, two things can happen. One, it draws energy from the Gulf Stream, often exploding into a new low pressure system just off Cape Hatteras, the point where the Gulf Stream and cool air are closest, and then intensifies into a classic East Coast Northeaster. Second, if there is a pool of cold air in place over New England and the interior Middle Atlantic states, the warmer ocean air does not penetrate as far inland, and also rides up on the cold air at a sufficient height that the resulting precipitation starts as snow and stays snow all the way to the surface. The further offshore the low tracks, the more likely it is to produce snow on land. However, the further offshore the low tracks, the less likely we are to have snow in interior new England and New York State.
Early in the winter season the conditions which allow for coastal and new England snow are more exacting, because the ocean water temperature inshore from the Gulf Stream cools down much more slowly than do land surface temps. Early season near-shore ocean temperatures can be close to 60 F. By February and March, the sea surface temps have fallen to 40 F and below. The early season Northeasters have to combine a near perfectly tracking offshore low and an especially strong pool of cold air, meeting in a perfect alignment. They do happen, but are not as reliable.
Not until February is the near-shore ocean temperature cold enough to allow snow to result from storms that track close to shore and thus can reach interior New England. That is why large snowstorms are more prevalent in February and even March.
All of this is controlled by "long-wave" jet stream patterns. The location and north/south amplitude of the jet stream largely determines how storm systems will form and where they will track.
Having said all this, I have to mention the other big factor... altitude. Many of the "mid-altitude" effects involved in the rain/snow question take place at altitudes of less than 8000 feet. There is no place in the east that is higher than 6000, and most of the ski areas top out at less than 4000 feet. While there is a noticeable improvement in snow reliability above 2000 feet, with the occasional exception of the upper levels of the Presidential range, all of the east coast ski areas are within the altitude bands that can be affected by warm, moist Atlantic ocean air.