According to this story in the New York Times, about a government campaign to replace 1950s American refrigerators, it's because of the "island's economic isolation, compounded by a United States embargo in place since the early 1960s." Oh, and "unlike education and health care in Cuba, refrigerators are not free."

And why would people replace their beloved American refrigerators with cheap, small Chinese models?

Cubans do not have to switch to Chinese refrigerators, but there are strong incentives to comply. When the exchange program is offered to a town or neighborhood, it is presented as the apple of Fidel's eye, and as an opportunity to show one's patriotism while lowering one's electricity bill.

UPDATE: Nope, despite the gratuitous mention of the free education and health care available in Cuba, no mention of "socialism," "collectivism," or "Communism," as having any effect on Cuba's economy. Nor is there any mention of Cuba's notorious (but apparently not notorious enough) "Neighborhood Committees for the Defense of the Revolution," nor, relatedly, what, the ahem, disincentives might be for failing to "show one's patriotism," though the author does acknowledge that at least one Cuban refrigerator-owner doesn't feel free to speak openly. And, for the unitiated, Cuba is not actually economically isolated from anywhere but the U.S., it just doesn't sell much of anything that anyone wants to buy.