The most important reason I can provide is that the country was enormous and there wasn't enough troops to effectively patrol it. As Lester W. Grau noted in The Bear Went Over The Mountain: Soviet Combat Tactics in Afghanistan, pages xii-xiii, "In Vietnam, American military strength rose to over 500,000 troops and the Americans resorted to many divisional and multi-divisional operations. By comparison, in Afghanistan, a region five times the size of Vietnam, Soviet strength varied from 90-104,000 troops. The Soviet's five divisions, four separate brigades and four separate regiments...struggled to provide security for the 21 provincial centers and...extend this security to the thousands of villages, hundreds of miles of communications routes, and key terrain features that punctuated and spanned the vast region."

In other words, at the height of their respective deployment strengths, the US deployed 25x more troops per area than the Soviets (5x larger force, 1/5th country size). Conversely, this means that Soviet troop "density" in Afghanistan was only 4% that of the US in Vietnam.