Piccolo sent Trunks and Krillin away...

[SSJ2]

To go destroy the single called Cell, yet wanted to keep Tenshinhan around to hunt down future Cell. Tenshinhan > Krillin, and even Trunks? Maybe Movie 9 was onto something.

 

[SSJ2]

Tenshinhan also noticed a disturbance in ki before Piccolo could, which is further evidence.

 

[Yoshi]

He might know about Shin Kikoho from Kaiosama's training.

 

[Papasmurf]

More like he doesn't trust the humans to get shit done so he kept the weaker one with him

 

[Let's Go Fearless!]

Because Tien is bigger than Krilin.

 

[withheldforprivacy]

Piccolo is secretly in love with Tien and was trying to seduce him.