The only way loons can get their heavy bodies into the air is to run straight into the wind on the water's surface, flapping their wings as fast as they can. Of course, in springtime the water is likely to become more open day after day, so this loon wasn't in serious trouble as it waited for more open water in other places. Lakes might be frozen and they couldn't land or find food. Risking the worst weather of spring, including blizzards, ice storms, and tornados. Eggs and chicks can be lost from other things, too, especially predators.
Early loons have more opportunity for re-nesting if necessary.
Best choice of nesting sites. Spending more time on the ocean means getting into top condition before migrating. When they migrate, the ice is out everywhere along the way so they can find food all along the way. The weather when they arrive is more predictable and warmer. One middle school class suggested that later migrants could maybe take the nests of the first.