The chief reason is decentralization.
Journalists who are updating these channels have their own sources in the Ukrainian air defense network as well as OSINTers who, for example, monitor Russian radio traffic using SDR, or even sometimes have people on the ground observing the take-off of planes in Russia and Belarus (horrifically dangerous, but there are ways to send this information somewhat safely; planes tend to be loud). If one of the journalists goes down for any reason, there will be other people writing updates. Each oblast also has their own channels where they announce attacks, some of them owned by the local administration, some by the emergency services. The air defenders themselves are a bit too busy to monitor and write this stuff; often, the best they can do is to write some short messages into a group chat or a Telegram bot before things go down, and even then, all parties involved have to balance providing an appropriate warning window with not letting the timing of this information to reveal the capabilities and locations of different kinds of Ukrainian observation stations. And this whole system has to be simple, since not every trained air defender is tech-savvy in general. Many don't know what an API even is. Many Ukrainians, too, wouldn't understand how to work with an API, but they can read the warnings in Telegram.
Also don't forget that the journalists who curate monitoring channels often also accept reports about the flight paths of missiles and drones from the general public, and while there are a couple of apps for that as well that send data from the phone's GPS and compass while the user is pointing the phone at the object, again, it's a matter of having several information channels that non-technical people can easily use. Even just writing to one of them that you just heard a cruise missile fly by, specifying your rough location, can be helpful, since radar coverage is not 100%. These messages then get relayed back to the people in the Ukrainian AA who are trying to intercept these things in real time.
Then there are the obvious security concerns, personal communications and group chat access can be vetted and it's hard to break the anonymity of Telegram channels from the outside to even be able to target the authors' devices with cyberattacks. While an API must be open to the world, and thus it immediately becomes a target.
It's a messy system but it works.
Kropyva is not available to the general public and it's very far from the capabilities of similar NATO systems, its strength lies in the fact that it's an Android app that can be used on cheap tablets, so it doesn't rely on the military-industrial complex provided hardware, which is safer and more robust, but far more expensive.
