They have not succeeded in spite of using every ounce of will power at their disposal. And no wonder, for will power is not enough and exerting mere willpower on the problem is too strenuous and nerve-wracking and avoiding homosexual activities just cannot be maintained for long. Some have never tried to overcome homosexuality because there are elements of homosexuality that they enjoy and they don't want to give them up. This is not odd. This is true of every sin.
Every sin has a kick and some thrill to it.
And to give up anything that has some enjoyment to it is not an easy thing to do. It does not matter if we have a desire to be free from homosexuality, we cannot bring ourselves to give up the fleeting thrills that it offers. Some continue in homosexuality because they have been brain washed to believe that they were born that way and that there is nothing they can do about it. Truth is not determined by mere feelings no matter how long one has had them. Truth for a Christian is determined by what God has to say.