While those are likely contributing factors, I think you're being a mite unfair on both franchises and sequels there! There are countless examples of sequels that far surpassed their predecessors, and, as mentioned, the X-Men property has produced far more quality titles than it has duds. Up until about, say, ten years ago, publishers could rely on a license to sell whatever godawful Turrican-a-like their rent-a-coders had shat out that afternoon, but - thanks in no small part to the phenomenal success of GoldenEye - I think that trend's been on a steady decline ever since. The Batman Begins game, whilst not a trainwreck, wasn't a Good Game by a considerable margin, and sold pretty poorly. The near-unplayable filth of the X-Men 3 Official Game shifted about eight copies. I don't know anybody who owns a copy of Ghost Rider, nor have I seen it in a single shop.
The good-but-flawed X-Men: Legends spawned the generally rather excellent Legends II, which in turn lead to the brilliant Marvel Ultimate Alliance, which arguably inspired the deeply average Justice League Heroes. Maybe it's DC's fault! They get to make the best cartoons, so Marvel get to make the best games.
