There are numerous reasons why LWW was made first, many of which have been already stated.
Firstly, it is by far the most famous, and most marketable.
It is also the best story to stand alone in case the others where not made (the others were only ever going to be made if the LWW was highly successfukl. which, luckily, it has been), and it is the best story to stand alone.
It is the book that was written and pub lished first (even though CS himself has said reading them in order of narnian chronology is perhaps better, he hgas also said it really doesn't matter, but personally, i think the order of publication is the most rewarding and best).
LWW is the catalyst for the chronicles. Narnia did not exist before the LWW, the chronicles did not exist, they arose out of the LWW, it is the most important book.
Also, as has been said, the MN is a prequel and thus gives that film (if made) more weight AFTER the LWW and others
the order I beleiev they will be made has been stated, and i agree with:
LWW
PC
VDT
SC
HHB
MN
LB
They will most certainly make PC next, as it is the logical sequel
I am confident that they will all be made and we will be with the cinematic versions of the chronicles for the next decade or so, and i for one am very glad