A question: was there ever a reason why people like Kankuro or Temari couldn't become Jinchuriki?

There doesn't appear to be any bias on age or gender, so Gaara didn't 'have' to be the Jinchuriki for the One-Tails, nor did Naruto have to be one for Kurama.

 

Gaara was the only one of the Sand Siblings who was compatible.

 

If someone is not compatible with the seal will eventually break and the bijuu will go on a rampage. That was what happened to the Hachibi jinchuuriki before Killer B.

How exactly Gaara was known to be compatible is a good question since he was still in his mother's womb when the Ichibi was sealed. Or maybe that's why he's compatible...

 

It really was never said what made Gaara, or B, for that matter, all that compatible, IIRC, so, for now, it seems that the reasons are arbitrary. A lot of stuff happens just because the author said so.

Personally, I like the idea that Gaara was really Sasori's child, and that was the reason why he was so different from his other siblings.

 

I believe compatibility is how well you resonate with the bijuu's chakra as Gaara was said to resonate well with Shukaku by Suna's councilors. Naruto likely resonated well as an Uzumaki and his mother being a jinchuuriki.

Anyone can become a jinchuuriki but how well you resonate with their chakra likely plays a big factor in controlling the bijuu's power. Three of the Kumo guys were judged to be incompatible and couldn't control Gyuuki. Age is also a factor since a jinchuuriki need to grow with their beast to adapt to them which is probably part of why those guys couldn't handle it. Naruto was a day old, Gaara was pre-birth, Yugito was two, Roushi was "young" and B was around five when they became jinchuuriki. And Naruto was able to synchronize perfectly with Kurama in the war because he spent his entire life with his chakra on top of his initial resonance. Tsunade mentions that it's unlikely someone else could achieve the same level of control that Naruto displays.

I think if you want a jinchuuriki as a weapon, an adult isn't suitable. Madara and Obito can likely get away with it because of their Uchiha/Sharingan and Senju/Mokuton genes, and Mito was apparently only meant to be container and had super sealing skills to boot.

Villages also use different sealing methods, as Cheddartrek mentioned, though I'm not sure what the actual differences. B mentions that Naruto's seal is of a higher quality of his own and Gaara's seal is probably what makes Shukaku's jinchuuriki into insomniacs to avoid being 'eaten'. I'm not sure whether making Gaara a jinchuuriki from pre-birth was because of the sealing method or an attempt to make him as powerful as possible, but I think his situation was unique.