Question

Why I can’t use table.sort to sort tables with associative indexes?

Answer 1

You can only sort tables with consecutive integer keys starting at 1, i.e., lists. If you have another table of key-value pairs, you can make a list of pairs and sort that:

function sortpairs(t, lt)
  local u = { }
  for k, v in pairs(t) do table.insert(u, { key = k, value = v }) end
  table.sort(u, lt)
  return u
end

Of course this is useful only if you provide a custom ordering (lt) which expects as arguments key/value pairs.

This issue is discussed at greater length in a related question about sorting Lua tables.