He lost [roulette] on purpose but for what reason who knows?
I think somebody already stated the reason he lost, so let me lay it out:
First off, let us call the man who he hypnotised, 'SubjectX'.
Now, we have to assume that the casino was fake. It was setup. It was probably a large room, with a roulette table, made to look like a real casino. This is the reason why he chose to use a VERY POOR camera.
He then placed a bet on the roulette wheel, with no practise (which is an insane thing to do, if he really was using timing, to predict the final result). In any sport or skill, people always warm up first and then start playing for real once they feel that they are "in the zone".
He lost the bet, by a single slot, which is incredible in itself (but I believe fake).
He then told us all that he would give back the £5k, which is fine because he never went to a real casino, so SubjectX's money was still in tact. No big deal.
DB isn't really out of pocket (apart from the cost of setting up the fake casino).
Now, lets assume once again, that the casino was fake. Only this time, DB guesses correctly and allegedly wins 170k Euros or similar. DB/Channel4, would now owe SubjectX, a substantial sum of money, which it would have to pay out of its own pocket because the casino won't pay out, as it doesnt actually exist.
Hence, the best way for DB to come out with some credibility, is to use a shakey camera, where the viewer can hardly make out what is going on. Then place a bet quickly (to prevent the viewers from getting a good look at the surroundings of the casino and point out errors/inconsistencies (which happened with his lottery prediction show). Then he managed to come within 1 slot of the casino. This allows DB to not be out of pocket and to come out with some credibility.
In the above way, I feel it is obvious why he did not guess the correct slot on the roulette wheel.