If there isn’t a difference, why didn’t the other four Justices in the majority join in his concurrence?

There’s a spectrum of opinions among legal conservatives on many decisions. Kennedy wrote Obergefell and Gorsuch wrote Bostock. But no legal conservative thinks that Roe was correctly decided and a fair number of legal liberals don’t either. (Alito quotes a liberal legal scholar in this opinion who said of Roe at the time that it wasn’t Constitutional law and didn’t even try to be.)

> If there isn’t a difference, why didn’t the other four Justices in the majority join in his concurrence?

Because commenting on other cases is generally not something the supreme court finds acceptable, heavily preferring to only comment on the questions directly before them.