Because, like so many AFL awards, it's a popularity contest...and it's awarded before the Grand Final. Clarkson is a modern day coaching great.
Is it awarded GF week or before?
Just looking at the past winners during the Hawks GF years;
2008 Bomber, I can understand as the cats 08 season was so dominant besides the GF.
2012 Longmire, Good year topped off by a flag but if the award is picked before the GF why was Longmire picked over Clarkson? The hawks finished higher.
2013 Hinkley, Port were deemed a basket case in 2012 and he took them to a semi in his first year so fair enough IMO
2014 Longmire, Sydney finished 1st so fair enough but how can they award Longmire the award in 2014 but not Clarkson in 2012??
2015 Beveridge, Similar to Port, exceeded expectations.
2016 Beveridge, finished 7th but made the GF before winning it so if it was GF week I can understand.