Why did the style of Mount St Helen's eruptions change from explosive to effusive?

the magma lost most of its gases in the initial explosive eruptions

Scientists realized long ago that no two volcanoes erupt the same. Some, like Mount St. Helens, burst violently and send ash and gas high into the air. Others, like Kilauea in Hawaii, ooze red hot lava which runs like maple syrup down the slope of the volcano. Many factors control how a volcano will erupt. Understanding these controls is a large part of the science of volcanology.