That's because Core updater is made to guard you against updating TB to version that does not support your current php version. Minimum for 1.5.1 is php7.4
So first update to TB1.4 and php7.3, then switch to php7.4, update to TB1.5.1 and iron out any issues, bugs, warnings, errors with your modules and theme.
After that update to php 8.0 - check once again for bugs, if none, update to 8.1 and you're super fast, secure (TB1.5.1 come with many bugfixes and security fixes) and happy.