Some cooks choose microwaving because it is much faster than roasting or boiling spaghetti squash. A whole spaghetti squash takes only about 10 to 12 minutes plus five minutes to rest when cooked in the microwave compared to one hour in the oven or 30 minutes to boil.
Since it is a winter squash, spaghetti squash is very hard and takes time to cook. Roasting the squash in the oven requires preheating time plus time in the oven. The microwave needs no preheating time and cooks the hard squash in a fraction of the time. Some cooks prefer the shorter cooking time to get dinner on the table faster.