It may have been racer-driven. The sail can be depowered much more effectively with the two loop. You can sheet really hard without bringing the sail in towards the centerline, which stalls the sail.
Back in the three loop days, racers would fasten the sheet to the cable between the port eye and the center loop. That way on starboard the bridle worked like a two loop to better depower on starboard when the sail is fuller. The two loop lets you do that on either tack. All of our sails, racing and recreational, are much fuller than the flat sails the boat was designed with, making the two loop more desirable.
I have a 1985 Sunfish "beach banger" (that I don't race) and it has the three loop bridle, and I will probably replace with a two loop bridle this spring (if we ever get spring here in Almost-Alaska-Upstate-NY.) My take is that the two loop rig is superior in almost all wind strengths, but especially in heavy air for all the reasons noted in this string. I had not thought about sailcraftri's point but he is right - with the load going to the center loop, all the load goes to one of the deck eyes, and most the the Sunfish that has the three loop bridle also have wood back-up plates under the deck eyes, and when they pull out, the repair is rather extensive. The newer boats that have a metal, threaded back-up plate under the deck eyes are a huge improvement and almost never fail. On my Sunfish project boats I almost always replace the three loop bridles with the two loop bridle.
3.7.6 Either one or both of the following bridles may be installed: 1. The wire bridle with a loop in the center, supplied by the Builder on older boats. The loop may be taped to one side to permit the mainsheet to slide the full length of the bridle. 2. The wire bridle without a loop in the center supplied by the Builder. The effective length of such bridle shall be 31 inches±1 inch (787.4mm±25.4mm) measured from eyestrap to eyestrap. The rope bridle, which was previously supplied by the Builder, is prohibited.