They did for a while, starting in the 1500s. It appears to have been a
hypercorrection—thinking that because “would” and “should” had Ls,
“could” should an L, too. But that was a mistake, because “would” and
“should” are the past forms of “will” and “shall”, which do have Ls
(although they are not pronounced in “woujld” and “should” anymore). But
“could” is the past form of “can”, and there had never been an L in it.
--
John W. Kennedy
Algernon Burbage, Lord Roderick, Father Martin, Bishop Baldwin,
King Pellinore, Captain Bailey, Merlin -- A Kingdom for a Stage!