Usually performance and storage concerns. You can set effectively infinite retention on Prometheus, but after a long enough period you're going to like querying it even less.
Most TSDBs aren't built for use cases like "query hourly/daily data over many years". Many use cases aren't looking further than 30 days because they're focused on things like app or device uptime and performance metrics, and if they are running on longer time frames they're recording (or keeping, or consolidating data to) far fewer data points to keep performance usable.
Most TSDBs aren't built for use cases like "query hourly/daily data over many years". Many use cases aren't looking further than 30 days because they're focused on things like app or device uptime and performance metrics, and if they are running on longer time frames they're recording (or keeping, or consolidating data to) far fewer data points to keep performance usable.