My non lawyer, but simple, and understood to be correct understanding is, you mainly have to show 2 things. First that you were affected. Second that the damages are real, and specifically quantifiable.
I really hate this decision, but an argument could be made that it’s too hard to identify exactly how Texas would be damaged by the actions of these other states, because the electors haven’t even voted yet, and if they do elect Biden, how can Texas see into the future for how that may damage them, then.
Courts are more for what has already happened, than what might happen. Like I said, I hate the decision, and think laws were broken and it should be corrected, but I am not a lawyer, just know it’s crazy how they think sometimes.
Trump joining the lawsuit should have helped the standing issue, since he would be personally harmed. They probably didn’t see his joining as actually legal though, since he is not a state.