People end up writing their own [1]. They rarely open source their custom monitoring infrastructure. Sometimes a private monitoring system gets open sourced, but then you see it has complex dependencies. The complexity of monitoring blocks wide-scale deployment. People stick with 15 year old, simple, dumb, solutions.
I'm working on making a new distributed monitoring/alerting/trending/stats framework/service, but it's slow going. One weekend per month of free time doesn't exactly yield the mindset to get into hardcore systems hacking flows [2].
[1]: http://www.slideshare.net/kevinweil/rainbird-realtime-analyt...
[2]: Will develop next-gen monitoring for food.