It's crowded because there isn't a clear leader.
For example if you want to store timeseries data long term and already use HBase then OpenTSDB is a good choice.
On the other hand if you want to do monitoring that's simple and dependable in an emergency with querying, graphing and alerting over short/medium-term data, then Prometheus would a good choice.
This in particular, but your entire post, would be a great addition to the Prometheus front page, or top of the documentation section. This wasn't clear to me initially, speaking as someone who evaluated prometheus a few months ago.
Ignoring that as it's planned work, the typical considerations are more around availability vs. consistency and that we're a metrics system focused on operations rather than a event store. Most of that's already covered in our docs. See https://prometheus.io/docs/introduction/overview/#when-does-... and https://prometheus.io/docs/introduction/faq/