Agronomic reasons: The soils here in the fruit belt are predominantly sandy loams. They drain well after a heavy rain or a rapid snowmelt event. But they contain enough clay such that they are not droughty. The sandy character allows for rapid root development. Most irrigation in this region is via trickle systems, where a perforated hose is placed down each row of trees. Water (and fertilizers, to some extent) trickles out of the hose, with little loss to evaporation and virtually no runoff. In some low spots, sprinkler irrigation is practiced, especially on nights when a frost could damage the crop. The warm water keeps the trees’ leaves from freezing, and even if they do freeze, the release of latent heat as the water turns to ice keeps the leaf warm enough to inhibit damage.

Climatic reasons: The climatic "lake effect" is strong here. The lake effect takes many aspects. First, it delays blossoming in the spring, because the lake, which is slow to warm up in spring, keeps the air near the lake cool. The cool temperatures delay blossoming. Fruit growers do not want their trees to blossom early because the flowers could then be damaged by the occasional late frost.
    The map below, which shows the date of last expected spring freeze, illustrates an important corollary to point #1: the last frost in spring is earlier along the lake than at inland sites of similar latitude.  Thus, the likelihood of a late spring frost damaging blossoms is minimized.

Source: Atlas of Michigan, ed. Lawrence M. Sommers, 1977. 

    Second, the first frost in fall is often delayed. Cold air, invading from Canada, has to cross the (now warmer) Lake Michigan. Cold air masses are warmed significantly as they cross the lake, and clouds formed by the exchange of heat and moisture from the lake moderate the temperature of downwind locations even further, significantly delaying the onset of cold weather in the late fall (relative to sites upwind of the lakes, or farther inland). The lakeshore area is a delightful place in fall!
    Third, the lake makes the hottest summer days a bit cooler. Fruit trees essentially "shut down" when the air temperature exceed 85 degrees F. These "extreme" high temperatures are not common near the lake, where the cool waters and associated lake breezes keep the air cool.
      Fourth, absolute minimum temperatures in winter are not as low as inland. Fruit trees generally begin to suffer damage to their buds at temperatures, depending upon the type and variety, of -10F to -20F. These very coldest periods of the winter are, again, moderated by the lake, which rarely freezes and thus, still has the ability to moderate the air at it crosses the open water.  The maps below show the number of cold (<10 and <15 degree F) days that one could normally expect each winter, for areas in SW Lower Michigan.   Note that, near the lake, the number of such "very cold" days is less than farther inland.  The lack of (or at least, limited incidence of) extremely cold days is a positive for fruit growers, since these very cold temperatures (found more often in inland locations) could damage the buds on fruit trees.

Source:  The Climatic Atlas of Michigan, Val L. Eichenlaub, et al., 1990. 

    Thus, to sum up #3 and #4, the lakes influence air temperatures by moderating (lowering) the highest summer temperatures, and by making the very coldest winter days not so cold.  In summer, the relatively cool lake waters cool the hot air as it comes into contact with the lakes.  In winter, the lakes release heat to the very cold air on winter days--warming the air nearby. 
    Fifth, though relatively unimportant, the deep snows associated with down-wind, lake effect sites, insulate the soil and keep it from freezing in most years. This can potentially be beneficial with regard to root damage, though it does facilitate the overwintering of some agronomic pests. Enhanced lake effect snows also provide a slow, effective recharge of soil moisture in the late winter and early spring.
    Sixth, lake effect lengthens the growing season--defined as the number of days between the last spring frost and first frost in fall.  The reasons why the last spring frost comes so early and the first frost in fall arrives so late are given above...and a map of the length of the growing season in Michigan is given below.

Source: Atlas of Michigan, ed. Lawrence M. Sommers, 1977. 

    Finally, and perhaps most important, the area has sufficiently rolling topography, with large local relief, to provide protection, on upland sites, from frost. Cold air, on calm, clear nights when frost is most likely, behaves like a liquid and drains into the low-lying parts of the landscape (see image below). These "frost hollows" experience frost on nights when upland sites may only get down to 40 degrees F.

Source:  Photograph by Randy Schaetzl, Professor of Geography - Michigan State University

Farmers in this region describe the landscape as a "good site" or a "cold site", depending upon its location on the uplands or in the lowlands. It is not so much the elevation of a site that matters, as much as its local relief and its elevation above a nearby lowland. Notice, as we drive, that the orchards are primarily on the upland sites. Many growers now use wind machines to stir up the cold, stagnant air that can drain into low areas on cold nights. The warm air, above the nighttime surface inversion, is mixed with the cold air in the lowlands, creating an atmospheric situation less conducive to frost.