I would also urge people to wonder: why do they care so much ? I don't have the answer. Especially in the past, it seemed like a cost factor to keep up a modern browser (just ask Mozilla) if people might as well install anything else and still buy your OS and use your services. That may have just been going with the flow, not wanting to miss the boat. My best guess for nowadays is the same as why Google hijacked a lo…
Microsoft Advertisement is pulling in 10 billion dollars a year (~=5% of total) and Microsoft wants to aggressively expand it.