For every other ability score there is at least one class where you can put everything in that score and have a viable character.
strength: barbarian
dexterity: rogue
intelligence: wizard
wisdom: cleric
charisma: sorcerer
Why is there no class for constitution? It's not just that it's never a key ability score. You simply can't build around it the way a rogue can build around dexterity. There aren't any benefits that normally balance out the fact that you neglected some stats.
If you want to make the smasher, who specializes in dealing powerful, smashing blows, there is an easy path to do so, and it will be a solid character.
If you want to make the sneaker, who specializes in deftly avoiding danger, there is an easy path to do so, and it will be a solid character.
If you want to make the tough guy, who specializes in outlasting everyone else on the battlefield, you have to make tons of twists and turns and it still won't be that good.
Why?