It's a fairly simple problem. EA gets money for you BUYING BF1942 and it gets money for you buying the expansions BUT it doesn't get money for you PLAYING BF1942 or the expansions OR any free/homebrew maps.
Therefore it is directly AGAINST the interests of EA to produce a level editor and free map packs. After all, the developers assigned to work on these projects aren't working for free, are they?
Now that you've bought BF1942, the release of expansions is the ONLY way that EA and DICE can continue to make money from their hard work.