When you put it like that you are absolutely right, although it's interesting, if you watch the show Derren went to great pains to get exactly the 'right' candidate.
Some of us are more susceptible than others, and he wanted to make sure that he got the most pliable person he could find.
While watching the show, I have to say that the more you listen to Sirhan, the more he sounds like he's telling the truth. Derren recreated the scene using his evidence, and he was able to recreate exactly what Sirhan described happened to him.
It just seems so inconceivable that he could have planned the murder, and then planned such a detailed and unbelievable alibi to give after he made no attempt to escape. Eye witnesses to the murder said that he just stood there in a daze, 'like he was in a trance or something'.
Not only did he give the same evidence at the time, he's still sticking to it over 40 years later. The MKUltra program was vast, it encompassed 200 different companies and organisations, including hospitals and universities.
It is so sad that they got this gentle guy to commit murder, and then to get away with it, while he rots in prison.
Derren set out to recreate this very specific set of circumstances, which greatly limited his options. There are other options. Sirhan Sirhan was selected, apparently with great success, from a far wider set of options than Derren has had to choose from.