Lots of reason: first and foremost, I think we've created a set of rules that surpase those in the MotP. The netbook rules for planes allow to create a wider variety of planes quickly, and are flexible and generic enough to create any plane imaginable.
Secondly, the MotP is not, and will never be, OGC. What we're trying to do is contribute something meaningful to the open-gaming community, such as it exists. The rules we've created can be used by anyone, whilst the rules in the MotP are very limited in focus, and can only be used by WotC.
So, please, download them and let us know what you think.