It may not be worded well but it does state why. Planets of this size are more likely to be geologically active, a sufficient size to retain an atmosphere thick

It may not be worded well but it does state why. Planets of this size are more likely to be geologically active, a sufficient size to retain an atmosphere thick enough for life, a strong enough magnetic field, etc.,. Just due to their size, they are better suited to support life then (at least as we know the conditions necessary). I mean between the other options - planets so small they're basically rocks in space and giant gas planets with a bunch of other issues - it seems this classification of planets indeed offer superior habitability.