Because sport by necessity operates on a set of rules above and beyond that of common law, enforced by contract. This is why, for instance, players don't get to sue referees for loss of earnings if they get sent off.
Look, I don't think anyone disagrees that the sports authorities occasionally overstep the line between "we must regulate this in order for the sport to work" and "we are above the rule of sovereign nations". Forcing the Brazilian government to sell beer in stadia so as to not piss off beer sponsors, for instance. And there are obviously specific cases where the law has to intervene in matters that the footballing authorities would rather deal with themselves (for instance, human rights issues regarding players: Bosman has fucked shit up for club football, especially smaller teams, in a big way, but it simply had to be done). But in terms of how the competition itself is actually run, it isn't at all difficult to understand why allowing local courts to make decisions which would override those laid down in the contracts that clubs sign would cause chaos. For all this talk of "mickey mouse courts" and whatever, Rangers broke a contract. If they were a normal business, you'd suppose that the courts would consider such a breach to be grounds for the SPL / SFA to summarily tear up their membership.