Of course, if one wants to maximize endurance while in a hold, flying at Vfto is strongly advisable. But this comes at a price: flying straight and level at Vfto will see pitch above +5°, which we are encouraged to avoid due to passenger comfort (our company considers the Dash a somewhat uncomfortable aircraft for passengers and therefore has provided whole libraries on how to somewhat mitigate the noise and vibration lebels). So while it would be possible to reduce below 180kIAS in a holding when weight and icing conditions permit, this is not normally done.
I would also assume that this is for standardisation reasons: when ATC calls several DH8 to fly Minimum Clean, this could result in speeds between 135 and 190kIAS depending on weight and icing conditions applicable for the individual aircraft. Also, when entering or leaving icing conditions, this minimum clean speed will change by the mentioned 20kIAS. All this has the potential to surprise the ATCO and mess with his sequencing. So Min Clean is 180kIAS here, as this is good for every weight and icing conditions as long as no turbulence is involved; if another, lower speed is required, it takes just a word from ATC.