The market segments are different... Disneyland gets a LOT of repeat business from the Annual Passholeders and locals, so by "adding" new attractions it gives them a reason to return during the holidays... so while your there to try out the "new" attractions, why not have dinner or do some holiday shopping.
At WDW, you have a also high percent of return guests but they are not the every couple of week Annual Pass holders, they are the once a year / once every other year type visitors. Most would not build a special trip around a re-themed attraction. WDW does do the longer festivals to help bring people in (Food & Wine / Flower & Garden). The Candlelight Processional running for the entire month of December or the Flower Power concert series end up being the "added" attractions for WDW.
My personal opinion:
Keep the HMH & SWH attractions exclusive to DL. As much as I am a fan of both (and the Matterhorn too), I don't want to see every Magic Kingdom park be the same. Keep them unique. If you really want though, we can send you Stitch's Great Escape
Edited By Tracks N Slabs on 1125976091