What limits Tenshinhan?

[super michael]

In-universe, I guess he's just been training wrong, whatever he's been doing ever since Namek just hasn't been giving him good results. Kinda like with Future Gohan versus Present Gohan, it's not about training a lot, it's about training right. It's hard to not go into out-of-universe reasons for this one, honestly, because even if I say "he should switch focus and try to unlock the myriad powers related to his third eye" as an in-universe statement, it still needs writers that are willing to expand on the concept. So in short, I'll just headcanon that his training method isn't working out heh for him, I think.

There is Present Trunks who is weak, since he is only allowed to train with Pilaf or Goten and we know Goten is normally not allowed to train.
Whis does mention that how Vegeta trains by himself, he wasn't going to get far. Vegeta powered up a lot thanks to Whis training and training with Goku.

So it isn't headcanon that training method is important.

Plus in the Cell Saga Goku and Gohan trained in a more efficient and smart way compared to Future Trunks and Vegeta. They both trained in the same location, but their idea of training was different, therefore the gains was different.

[MasenkoHA]

There is Present Trunks who is weak, since he is only allowed to train with Pilaf or Goten and we know Goten is normally not allowed to train.
Whis does mention that how Vegeta trains by himself, he wasn't going to get far. Vegeta powered up a lot thanks to Whis training and training with Goku.

Weak in relation to who? They're stronger than any other character was at their age.

[super michael]

There is Present Trunks who is weak, since he is only allowed to train with Pilaf or Goten and we know Goten is normally not allowed to train.
Whis does mention that how Vegeta trains by himself, he wasn't going to get far. Vegeta powered up a lot thanks to Whis training and training with Goku.

Weak in relation to who? They're stronger than any other character was at their age.

Gohan at the age of 9 he was stronger than Cell and also had SSJ2.

Goten and Trunks on the other hand are weaker than the other Z fighters such as Goku, Vegeta, Piccolo, Gohan and Boo.

[Izanagi]

There's literally nothing that stops Tenshinhan from getting stronger. Humans now have so many different ways of getting strong and catching up to the Saiyans from the Ultra Divine Water of Karin, from the Grand Elder's ability to draw out one's hidden potential, and the best one from the Old Kaioshin. They could even just do what Piccolo did and ask Shenron for a power boost. And if nothing works, then they can just go transhuman and become an Android to close the gap with the Saiyans considering Bulma's dad has Dr. Gero's blueprints. The only thing that keeps humans from going that route is pride.

Also, people complain about the humans being weak as shit, but this is the real kicker: When you look back on things, the humans had massively greater gains than Goku in far less time when they did the same training as him. Humans went from being weaker than 23rd Tenkaichi Budokai Goku and Piccolo to being nearly as strong as Raditz in one single year. That's like thrice as stronger as Goku got in three years in a third of the time. Actually, it was even less than a year because Kami sent them off their own way before a full year passed.

What gains is Tenshinhan gonna get by being holed up in the mountains with only the white midget as a possible sparring partner doing the same training he's been doing for decades now? Of course he'll remain piss-weak. Not just that, but Kuririn and Yamcha also refused to train with Goku and Piccolo after Namek when offered, and as it's consistently proven, you gain more out of your training with a sparring partner.

I will maintain that if the humans actually sought and did proper training they would be able to be true contenders. But they never do, and so remain useless and weak. But also, the real is answer is just that DB is the type of series whose story decisions are dictated by what sells rather than what makes for airtight storytelling so the humans won't ever be allowed to share that experience.

[Zephyr]

But also, the real is answer is just that DB is the type of series whose story decisions are dictated by what sells rather than what makes for airtight storytelling so the humans won't ever be allowed to share that experience.

Another out-of-universe explanation is that Toriyama only juggles so many main cast members at a time. There are always new strong guys getting added to the cast rather than keeping the same consistent stable, so it kind of turns into a revolving door of companions and rivals for Goku. I think Gaffer Tape made the point in one of his Saiyan arc videos that Tenshinhan and Chaozu were basically replaced by Piccolo and Gohan, and I can definitely see that.

[Yasai9001]

I may be a little late to the party, but to simply put it, it's because Tien was tainted by Tsuru-sennin's (Crane Hermit Master) evil guidance - it hindered not only his potential, but special powers that would have naturally came to him had he chose a path of righteousness from the get-go.

[FoolsGil]

Semi-Retirement. Others have said it, but he has no interest in doing whatever needs to be done to keep getting stronger. He fought for the planet multiple times, and his Kikoho can wipe out anyone weaker than Semi-Perfect Cell, which is not a small feat in a universe where Pre-Super Freeza was the strongest being. He is a Super Human, even if he can't beat Goku - That's good enough for him.

[Cybersai]

It's pretty easy to see why, it seems the last "major" training he did was for the 3 year wait for the Androids and who knows what he was doing.

1. He didn't go in the time chamber to train before the Cell games because he admitted there would be no point.

2. The 7 years of peacetime before Buu likely had him taking it easy like most characters on Earth, just doing basic martial arts sparring here and there.

3. During the early Super era, he didn't have much reason to train when Beerus came to Earth, helped out against Freeza's henchmen

4. Didn't go to the U6 tournament.

5. No role in the Zamasu arc.

6. Kinda got screwed over in the Tournament of Power.

So what did people expect really.