After the Jacobite rising of 1745, the Dress Act of 1746 banned Highland dress, tartan included, for anyone other than the military. It stayed in force for thirty-six years.
When the ban was repealed in 1782, tartan came back not as everyday Highland wear but as a symbol — worn deliberately, and increasingly by people with no Highland background at all. That shift from clothing to symbol is most of why tartan means what it means today.