Great question! Long term data is stored, but the website hasn't evolved yet to display that information.
As a temporary work around, I've been taking the URL for the Las Vegas ASOS (you can use any western region ob) that capture the 5-min observations (https://www.weather.gov/wrh/timeseries?site=KVGT), then switch out the airport code for the MD mesonet stations, which use the nomenclature of three digits then MD (EX: 001MD, 002MD, etc.) Once you swap out the site code, refresh the page then left click 'Advanced Options' and you can select a time from the 'Number of Hours to Retrieve'.
Paging @wxmeddler in case I missed anything.