Mike
Over the years i have often wondered the same.
However It is all to do with location and conditions.
I have found that certain areas of the river will have trout willing to rise to MB's as well as dark olives. It is IMHO more to do with water conditions.
When the water is the colour of irn bru a few days after a wee spate then the fish will go daft for MB's, once the water temp rises a bit they will rise too, unfortunatley the MB's are often long gone by the time the fish are fettled to rise, last year being the best exception for a long time.The flies delayed till may and fish were more than happy to rise.
That said my best day on the don was April the 1st 1989, i have never had another april day the same since, despite searching for it
I cant really remember what the winter was like though, but i do remember every fish
I would look for a deepish flowing (riffle) stretch about 50-100yds downstream of a shallow fast pocket style water. I would sit down and wait (approx 1200hrs-1300hrs), when the flies appear the fish usually follow about a minute or two after, be ready. It 'll last about 20-30 mins and then its over. Then its search fishing, create your own hatch with casts over the same area, it can work.
I also dont think this is confined to Don fish, i have seen it on the Ythan and Deveron as well as the Isla, ericht and tay. Its just broonies
And Fred if you go down to parkhill you will see the biggest hatch of LDO's you'll ever see anywhere, its truly amazing,the water is litterly carpeted with flies and again not a fish to be seen anywhere.
keep trying though, the days it happens are worth the wait.
Sandy