It's the only edition which is provided for free. This edition uses Lua 5.0, which is very similar to 5.1 which is still very popular as it's targeted by LuaJIT. The changes between 5.1 and 5.2 seem small as well.
> The changes between 5.1 and 5.2 seem small as well.
And then after 5.1, basically every minor version (5.2, 5.3, so on) have major changes between them, from what I recall.
Of course, there is also this (quote from the site):
> If you find this online version useful, please consider buying a copy of the current edition. This helps to support the Lua project.