Interesting question, yes
Besides above answers about waves etc - which most agree about it seems, there is another thing:
Riding a directional in flat water or chop or waves - does not matter where, it just FEELS so much better in every way IMO
And a mutant is a board that is equally bad at freestyle and waveriding - a thing in between a TT and a Waveboard one could say ?
And a Twintip is also a mutant somehow, as aa TT is a board where you go on compromise with performance, by having both ends equal - which is NEVER the best for anything - except for doing both "sides" equally bad
So in order to do freestyle and land backwards (and for beginners to learn to go the other way without turning), we use the least efficient board shape of all boards - the TT
A mutant (a classic one) is more performant than a TT.
A raceboard is more performant than a mutant.
A waveboard is a raceboard which has been "throttled down" a lot, for maximum flow and agility with softer lines and fins.
So for me it is the feel of riding a directional (no matter what type) that just feels and seems so more right in every way
No compromises at all with a directional, and no downsides
, no awkward stance because you have to go on compromise in both directions like on a TT
So if it was'nt for typical freestyle fun and landing both sides - I would not own a TT at all, as if you have a waveboard and a raceboard and a classic mutant you would be able to do everything better than any kind of TT board: You can turn faster, start earlier, jump higher, ride faster !