My mother in law passed away on Christmas day, 2003. She didn't want a funeral so we had viewing hours at the funeral home in December on the 27th. Her burial was in May. She was buried in a cemetery from the late 1700s/early 1800s which is hard enough to navigate in good weather. They weren't able to use big equipment to dig her grave. We had a private burial. I took my bereavement time in the spring because I was on break at Christmas time. It was tough to say goodbye twice, but that's "how it's done."
Someone else mentioned the frozen ground/mud aspect. I live in rural, central, western Maine...rocky soil, ground frozen solid from November through March-April, and in early spring it's very, very muddy. Roads are posted during Mud Season so no heavy equipment can pass over them in hopes of preventing damage to the roads. In winter, the ground is so frozen that, yes, you can dig a hole with heavy machinery, but it's very difficult and costly to do so. All the cemeteries near us are closed to vehicles all winter.
The "cities" in Maine (Portland, Auburn and Lewiston, Bangor, Augusta) may have bigger cemeteries that do winter burials but the smaller communities likely do not. Heck, the one church in my mom's hometown is closed all winter because it's too expensive to heat. People have to go to church in the next town during winter! Funerals there are held at funeral homes or in the neighboring town's church during winter.