Well the I group was probably the earliest to settle in Europe which is the best continent for living conditions and they didn't worry about migrating anywhere else, they got comfortable and might've killed off the neanderthals.
The R1b came in later and then the R1a came in after them. In larger numbers probably which is why they are the most common today. But the R1b got pushed out further west over time to the ocean, not being able to expand continentally they expanded through the seas and this is how the British conquered or colonized a lot of the world. The other ones like the Spaniards, French, Dutch, Portuguese did as well but they pretty much copied the British. And the British got invaded by the Vikings through the sea that probably gave them the idea they could do the same. And the Germans have a lot of R1b also but didn't expand much, and when they did again probably tried to copy the British.
These sound like sloppy theories, yes...
Btw Hitler and Stalin or Genghis Khan, Alexander the great or the Romans, Vikings, Greeks, Persians, Egyptians were not all predominantly R1b. Civilizations come and go dude.
It's more interesting to me why are the Rs bigger in populations and other hgs that are highest in number... environment? luck? more horny? Are there interesting scientific studies on this subject?