Because its easier to pronounce ого as ово, thats it. Look at your own languge: why dont you pronounce all letters in words like knight, thought?? So we have the same feature, as pace of speech increased people started to replace 'г' sound with 'в'... and i think the same thing happened to 'о' and 'а' (молоко became [малако])... People choose ways that need less effort.