Maybe not the real reason, but in my opinion, because he exemplifies what is wrong with the Jedi and what leads to their downfall. I talked about this in the Ode to Qui-Gon thread.... He is basically very agressive, and not in tune with the force. He is more a politician than a Jedi, as are too many of the Jedi, or at least in some ways.
Spoiler:certainly is a wonderful symbol of this, because he is probably the worst offender when it comes to this sort of thing.
His being the first Jedi to go (more or less)
he's an expendable character and has been since the moment he was introduced.
but seriously, try to come up with a better way for him to go. if he takes it from a saber, thats too predictable and as 'the second strongest' jedi, pretty lame.
plus I think its a nice mirror between the OT & PT. anakin watched as Mace fell, covered in lightning & that was more or less, his turn to the dark side. then anakin was the one who finished the emperor, who fell, covered in lightning and that was anakins turn back to the light.
Since they from what i hear just show him falling not his actual "death" I think it was left open for some novelist to make a few bucks by writing a book in bewtween 3rd and 4th episodes showing maces return and demise again...lol....probably tries to take on an apprentice in hopes of defeating the sith since he cant do it himself all crippled up what not and then he tries to teach em vapaad and they become a fallen jedi dark jedi what ever....then stick it to mace for an even less honorable death...lol....yes yes yes MACE DIES AGAIN! oh yeah also I think Obi or Yoda shoulda went back and graverobbed that amethyst lightsaber!! That was the coolest saber I thought...Maybe in this future book I mentioned Maces apprentice will have it....and use that one to stick it to mace....lol
um.... nope... well maybe.....
there's a thread about her somewhere. She mentioned GL had spoken to her about doing the TV series.
http://www.killermovies.com/forums/showthread.php?s=&threadid=338281&highlight=Aayla+Secura