The guy who started the thread has shuffled off this mortal coil, gone to the happy landscape place in the sky where mowers never break down, gas is under a buck a gallon, and all customers are nice and bring out lemonade and cookies at 3:00 on hot days.
Of the remaining:
A good fraction are out mowing lawns and otherwise earning their biscuits.
Some have quit the business. Lawn service companies have a short lifespan.
Some have lost interest in the forum. They forgot their login or their password, and it wasn't worth the bother.
Most forums have a 6 month half life for members. Of a thousand who join, 250 are still active a year later, 60 two years later, 15 three years later and so on. Half drop out every 6 months.
In the batch of posts just on this page, we have 1 guy from 2006, one from 2009.