I believe it is a Garmin Connect limitation.
You can contact support@xertonline.com for a definitive answer. They may also be able to work around it.
Typically, you don’t need more than a year or two to effectively use Xert and compare this year to last year, but I understand you’re looking for a longer-term trend to analyze.
Lots of newbie tips posted here – Beginner questions
Academy series videos are the quickest way to get up to speed – Discover + Improve + Perform
FAQs here – Support Home – Xert
Welcome aboard. 