I think the biggest issue is the shear number of coasters "enthusiasts" ride, and the number of re-rides we will have during a year.
For the majority of guests, they maybe visit a park once or twice a year at the most. From those visits, they'll tend to spend more time "faffing" or soaking up the atmosphere. It's a treat so they go to enjoy it.
If you ride one rough coaster a year, you tend to forget how rough it is. Your excitement level of riding a coaster out-weighs the pain it gives. That doesn't mean people don't notice roughness, it just means considerably less to them.
As an enthusiast, by the time your spine has been hammered out through your skull for the seventh time in three weeks due to a succession of rough woodies and SLCs, you tolerance drops. You are getting less from a ride (particularly if the ride is pretty mediocre), but you're feeling the pain more. The pleasure/pain balance tips the other way from an infrequent rider.
I don't ride Rita often because it hurts me. Yet I'll usually ride once or twice a season. Yep, I'll complain about the pain, but I also enjoy the ride. If I rode it every time I visited Alton, I'd just associate it completely with pain and very little pleasure.
There are some exceptions... Baco I have no desire to ever ride again due to the serious amount of pain it caused me. For the most part though, again, I'm riding a lot of coasters, so more on the look out for the pain issues but getting little "thrill".
It's just simply because we're completely immersed in the hobby it affects us more. Of course, some people don't mind getting beaten up - we all have different pain thresholds after all...
