Monk weapons continue to be a weird pain point in 2e that feels like a remnant of older editions.

Monk weapons are in a bit of an awkward space in that their main purpose is to make sure that monks have access to a diverse array of weapons that match a variety of popular themes, but then there's some kind of uncomfortable orientalism that occurs when every Asian-inspired weapon has the monk trait tacked on. It's one of those reasons that I try to watch for opportunities to introduce non-Asian monk weapons that still make sense for the martial artist theme, and to try and introduce one non-monk Asian-inspired weapon for each new Asian-inspired monk weapon that's introduced, but it's just hard to do without introducing things that rub some people the wrong way when the options don't match their image of a monk, or when the option isn't the most optimal one for the monk specifically.

Flurry of Blows bars monks from having full martial proficiency (basically every limitation in a monk feat or ability referencing agile/finesse/monk weapons is there to make sure you never get to break the damage ceiling by flurrying with e.g. a greatsword or similar weapon), and a closed list in the class itself starts sucking basically as soon as you publish the second book in the edition (look at rogue and wizard weapon profs, or the various alchemist abilities we recently did errata for to open them up and give more player agency.)

So the monk trait does serve a very important purpose that goes well beyond just legacy pickups. You could just make it so they can flurry with any agile or finesse weapon, but then you'd lose a whole swath of non-agile/finesse weapons that match the theme and are still appropriate. You could just not do the weapon-wielding monk, but it's a concept that exists in our game world and has been popularly riffed on by franchises like D&D, Final Fantasy, Shadowrun, and literally hundreds more, so if it wasn't there, people would be asking where it went and when they're going to get it.

Personally, if I'm around for the next full edition cycle, I think it'd be worth re-evaluating how the monk trait is deployed and retooling the class to be less "Shaolin, specifically", and more "martial artist, generally", so that you can bring in a wider array of martial arts weapons that include things like hatchets, shields, maybe even certain pistols. But that would require a much more significant rework than is likely immediately apparent.

For the khakkara specifically, since it was put in the same book as the oracle and presented as the iconic oracle's primary weapon, I'd bet that what happened was that the weapon was designed to be more appropriate for a traveling priest than a martial artist, more of a straightforward beatstick. The version you'd want for using it like it's used in Shaolin styles would probably be a lot more like the whipstaff, with a smaller damage die but the ability to parry and make sweeping attacks.