Because it's *almost* going to do away with the M3 name, that has been around for 27 years. The only way you'll be able to have the legendary M3 is by buying a saloon, which hardly anyone does, over the coupe and convertible. In 2009 only 144 bought the M3 Saloon, whereas 1080 people bought a Coupe/Convertible, which will soon be known as the "BMW M4". This means it is going to be one hell of a rare thing to see a new BMW M3 in the future.
I know it's just a name but it is one of the most recognisable names in the automotive world that they're messing around with. It's never been a problem for the last 27 years, so why have they suddenly started fiddling? The only reason i can see why they bothered to create the "4 Series" was just to fill out their range and have a stand alone competitor with its own name to rival the Audi A5 - ie. Marketing . Unfortunately, this means almost doing away with the legendary "M3" name and i really don't think it is worth it.