Because most refs in football, as I said, are poorly trained to make the right incisive judgements, so they already use yellow cards for everything from 'mood setting' to serious specific sanctions. Give them a second 'safe' card and they'll soon turn that into more of a capricious gesture than a genuinely consistent and pertinent response to a foul.
I don't recall talking about a 'sin bin' as such. I don't feel completely opposed to the concept, but I have major doubts that it would suit football. For one thing, I suspect sports scientists would question the prudence of using that kind of arbitrary break for high intensity athletes in a 90 minute game; it depends how long the break would be. But I'd have thought a stop-start process might breed injuries. It would certainly wreck someone like Studge. And in terms of a spectacle and a sporting event, I'm not sure the majority of people would find the system more coherent than chaotic.
But I repeat about refs - you can either improve their training to enforce a simple set of rules, or you can over-compensate for their shortcomings by adding many more rules. The yellow card was mainly introduced (aside from making decisions more visible of course) in response to a feeling that refs couldn't be trusted enough to decide when a player deserved to be sent off, and also players couldn't be trusted to listen to a ref's warning(s) about their behaviour, so Ken Aston proposed the process: 'Yellow, take it easy; red, stop, you're off'." It was also in part a commercial decision, as broadcasters had complained that too many games were being spoilt by stars being sent off for dubious reasons.
A third card, for me, is just the wrong way to look at it. Instead of making more of an effort to teach refs to read situations more shrewdly - and drafting in ex-players as refs would help that - it would basically encourage them to think: 'Right, I've now got TWO goes before I really HAVE to get it right'. Refs should be doing less, not more. Making players more mindful of the rules, as well as refs, is the best way to enhance the game's simple core appeal,