Fly rods for streamer fishing are generally built to be tough and cast large weighted flies well, without damaging the rod. Anglers fishing for predatory species like big trout, bass, pike, musky, or other meat eaters will often be fishing with heavy line around structure which can result in snags. Removing these snags is really hard on a delicate trout or even a steelhead rod which simply aren't designed for that type of work.
Streamer specific rods are sometimes shorter as well for delivering reasonable length casts into structure or wind with great accuracy. Since you won't be mending either the additional length of a traditional rod is simply not necessary. Hard hook sets with a short rod and accurate reasonable length casts are the name of the game here.