It is current design spec.
60 days of metrics are kept by the API portal.
Older metrics than 60 days are purged.
The number of days to keep is configurable via the property PURGER_NUM_DAYS in /opt/Deployments/lrs/server/webapps/ROOT/plugins/lrsgateway-conf.xml.
Add <Property name="PURGER_NUM_DAYS" value="180" /> to keep 180 days of metrics instead of 60 days.
A portal restart will be required to activate the configuration change.
There is no way to regenerate the information that has been purged.