Why is there a lower limit on amplification for this filter design method?
I don't see either why it is a requirement.  However, you can achieve a 2nd order LPF with attenuation without an an op-amp, and without introducing power supply noise, dealing with input bias current noise, and the other few limitations of op amps.  

From TI's Design Methodology for MFB Filters


  Using an active filter for a Q &lt; 0.5 (two real poles) and/or gain &lt; 1
  is unlikely because a simple passive circuit can easily provide
  attenuation and two real poles without requiring an active element.


At this point active vs passive really depends on the application requirements such as cutoff frequency, Zin, Zout, area, cost.  Additionally, the typical differential passive LPF will have a differential output, while the active circuit in question has the output referenced to ground.