Why does numeric for loop not accept an explist?

["Peter Cawley" <lua@...>]

Generic for loops can use either a function call or an explicit
argument list, like so:

for key, val in pairs(t) do
  print(key, val)
end

for key, val in next, t do
  print(key, val)
end

However, numeric for loops are restricted to an explicit argument
list, so the first example won't compile:

function indicies(t) return 1, #t end
for key = indicies(t) do
  print(key, t[key])
end

for key = 1, #t do
  print(key, t[key])
end

This is exactly what the complete syntax specifies:
for Name `=´ exp `,´ exp [`,´ exp] do block end |
for namelist in explist do block end |

The complete syntax does not give the reason for this, so what is the
rationale behind this decision?