Sometimes, particularly in disaster situations, the most help you can give is to stay away and let the people who
can do the most good work without distraction.
The original intent of the rescue, judging from the Washington Post version of this article, was to have the two rescue drivers carry the kid between them as much as possible. It was realized when they were transporting the first kid that said transit mechanism was impossible, as the space was too cramped to do that; the operation then had one diver carrying the kid, swapping divers frequently because that's a very physically taxing job.
Between the weight and the length of the sub, it's clear from descriptions of the rescue operation that it was completely unusable. Any attempt to use it would have had a very high risk of it getting wedged somewhere in the cave that would have made the cave impassable. And guess what? Pretty much everyone who had some sort of experience with caving commented that the sub idea was unworkable when it was first announced.
Just because you want to help doesn't mean you can help, and you shouldn't give someone a pass on their actions just because they're trying to help. As the saying goes, the road to hell is paved with good intentions.