Why isn't the sticky bud plant more widespread
As already pointed out in comments, Galium aparine is actually very widespread, according to wikipedia it occurs in "a wide region of Europe, North Africa and Asia from Britain and the Canary Islands to Japan. It is now naturalized throughout most of the United States, Canada, Mexico, Central America, South America, Australia, New Zealand, some oceanic islands and scattered locations in Africa."
Because it occurs on moist and rich soils, it doesn't occur everywhere. It is unable to grow on dry and nutrient-poor soils (like deserts and steppes). But it is also absent in frequently mown grassland.