Good question, and here’s my explanation for why:
Braid is of a very thin diameter and because of this there’s a danger of its misuse as a mainline. Irresponsible anglers can be tempted to use it in very high breaking strains and to fish in very snaggy / heavily weeded areas. If a fish were to get snagged and/or the angler in question was to pull for a break then the fish could be damaged.
If used properly and responsibly it is fine and can be very effective due to its no stretch properties, it gives you a better insight into the lake bed and a real sense of being in touch with the fish when playing it.
Personally it’s not my preference to use as a mainline as I prefer the forgiving nature of mono – generally it results in far less hook pulls at the critical moments.
All the best,
JB & Team Crabtree