Up through 1992, both the Winter and Summer Games were held in the same year, on identical quadrennials. For Olympic fans, that sure made for an exciting and busy year, with two months generally 5 or so months apart where the world's best athletes gathered, and one could with entertainment compare and contrast the relative pros and cons of the various winter and summer sports.
From a practical point of view, the separation made sense. The larger base of casual Olympic fans wouldn't be overtaxed with attention demands, and the Winter Games were freed from a restrictive sense of prelude to the Summer Games. As well, as a large bureaucracy with travel and oversight demands, splitting the Games eased on the schedule for the IOC themselves.
But for all that, I'm guessing that a larger reason was simply money. With the Games separated, the Winter Games could reasonably be expected to attract larger broadcast and sponsor dollars, in a year where those dollars aren't directed toward the Summer Games. And judging by the amazing financial success that both the Games have had since for the IOC, it certainly looks like the right decision all around.
The 1994 Lillehammer Games were the first Winter Olympics to be held on the new cycle, and came just two years after the 1992 Albertville Games closed out the old cycle. Lillehammer is largely viewed as hugely popular and successful as a Games host, and, thanks to some particularly noteworthy events (er, notorious), did well to justify the hoped-for popularity of the Games without the larger Summer to follow.