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Why does anyone care what an excommunicated priest said about theology, or about anything, really? He was not authorized by the Church...

Well first off, many believe that John XXIII was a valid Pope and as such his excommunication was lifted. Others believe that his excommunication came not from from Pope Pius XII, but (if I'm not mistaken) the Holy Office, which was part of the same structure they believe got turned inside out just years later at Vatican ii. Still others believe that the Church was becoming increasingly lax on the salvation dogma (culminating at Vatican ii) and such an excommunication was merely an effect of that, and as such can be treated as if it didn't happen. 

In short, he went against the grain, in a big way, and so attracted a lot of attention. As you probably know, there are some Priests who will deny communion to those who hold his beliefs so it is not only his followers who care what he thought. 

Also, no trad clergy can really be "authorized" by the Church until we get a Pope. Sure there is supplied jurisdiction but there is currently no hierarchy that operates on the basis of giving imprimaturs or clearing up current theological controversies. Such is the nature of the crisis. 

(Sure this was tongue and cheek but so was the question)

+ABL was excommunicated for theological disagreements with V2.  (And if new-Rome wanted to, they could easily excommunicate every single Trad cleric in the world).  Ergo, using your flawed logic, why should anyone convert/follow Tradition if they aren’t approved?

Because the Vatican ii anti-popes and their henchmen have no right to approve of or disprove of anything on God's behalf.