Sources indicate a couple of reasons for that: First, the new 3 Series (pictured above in M Sport guise) on which it'll be based has so far has only come out in sedan form. Second, because the next-gen M3 is anticipated to lose some of its edge. Which would be a shame, but there's a far more straightforward reason than either of those two: there will be no coupe or convertible M3 from here on out.
Well, sorta. There will be, only it won't be called the M3. It'll probably be called the M4, since BMW is said to be spinning the two-door version of the 3 Series off into a 4 Series – more closely mirroring the 5 Series sedan and its coupe/convertible counterpart, the 6 Series. Since BMW has yet to unveil the 4 Series, we're likely to be looking at an M3 sedan first.
We expect it'll have a twin-turbo six-cylinder engine in place of the outgoing model's naturally aspirated V8, but the jury's still out on whether those pistons will be arranged in a V or a straight line.