I have a Mac desk top OS Yosemite 10.10.5 and want to upgrade the computer? Why Can't I do this?

Do you mean an iMac? If so, what is the model year?

See:

The model year of an iMac is available at Apple menu > About This Mac.

A late 2009 through mid-2011 iMac can be upgraded to High Sierra 10.13.6 but no later version. See: https://support.apple.com/en-us/HT211683

A late 2012 through late-2013 iMac can be upgraded to Catalina 10.15.7. See: https://support.apple.com/en-us/HT211683

A mid 2014 and late 2014 can be upgraded to the current version of Big Sur. See: https://support.apple.com/en-us/HT211683

A late 2015 or later can be upgraded to the current version of Monterey. See: https://support.apple.com/en-us/HT212735