What about THF makes it able to solo?

Over the years I have seen comments often like (soloable by THF/NIN). What is the trait that makes this something THF is good at?

Depends on the era in which you're talking.

THF has good eva, which obv helps solo certain things.

75 bloody bolts were a large help, the ability to sleep bolt and TA NPC was also lovely.

Abyssea, kinda goes without saying.

Currently, glasscannon THF excels at killing the enemy before it kills you.

I would say something to the lines of: "The players who use THF are of a higher caliber than your standard issue player." But there are Thieves among us that make us cry, so.

I think TH may also be partly to blame. Of all the jobs you could try to solo something with, I imagine it'd be tempting to try on the one that might offer a slightly better drop rate. So it likely has a larger sample size of attempts.