Because 1.27 is still as a standalone Patcher available. There is still over a special side service the option to download the game Classic Warcraft III --- Official Installers - Official Patches
and it is in 1.27 The problem is, 1.28-1.31.1 where made with the new patching system so there is no option to download it anymore because you will be instead lead to Reforged. They even took down the option to still download patch 1.31 from the PTR Server.