Well, basically because of the amount of money he cost us. Not just because of the signing on fee, but in terms of wages, the treatment we would have paid for when he was out for long spells with injury and we knew that even when he was fit he was gash. Shouldn't have been bought in the first place, but for the manager to perservere with him just smacked of pig-headedness. The last part obviously isn't Byrne's fault, but he was a near hopeless defender. Had a couple of ok-ish games I'll grant you, but for the amount of time he was at the club and the amount of money that must have been wasted a couple of ok-ish games really is nowhere near good enough.
There were worse players than him undoubtedly, but when it comes down to the reasons above he only edges out Jackie McNamara due to length of "service".