Yeah, why would you use a tested system that just "works" if you could add some npm dependencies, node.js and a 3 month old database that is out of support already if you are not upgrading every week?
Zabbix dates back to 2001 and is written in C, PHP and Java. [and in my experience of it, has the typical monitoring system problem where the design seems to be "the more panicky alerts it spews, the more value it's creating"].
Because it just works. Grafana requires a lot of work to set up, especially if you want to do it in a secure way and provision it programmatically. Munin on the other hand is just an apt-get install plus some minor configuration. Yes, it is not as powerful but on the other hand it is much eaiser to set up correctly.
It just work, it is easy to deploy and easy to write plugin for.
A combo like prometheus+grafana and other is very good too and definitely more modern. But, having used both, I often didn't need the extra feature that the more modern solution offer and I was more than happy to use munin, which was definitely easier to run and maintain (for me).
I use(d) both. While I appreciated Zabbix's configurability, it just takes too many clicks to add a basic probe with an alert or a new system. Meanwhile Munin has everything in flat config files (easy to deploy with ansible or equivalent), and has reasonable alerts configured by default.
And creating a munin plugin is as simple as writing a script that writes values to stdout (in whichever language you like), and copying it to /etc/munin/plugins/.
The advantage of zabbix is the templating and service discovery. You shouldn't have to add a new system to anything, it should just be found and monitored. The same goes for prometheus.
Monitoring new things with zabbix is also "write a value to stdout". Using telegraf and prometheus you have the same flexibility. I think these days you can even scrape prometheus exposition with zabbix. There are plenty of footguns with prometheus and zabbix, but they take big scale to run into.
But at the end of the day use what you're comfortable with.
It's lightweight, it's flexible, it's hackable and it's very orchestrate-able. I've had dashboards showing summary stats for IoT devices, the VMs they're talking to and the hypervisors the VMs are running on with very little effort.
