Jenna got her first boobjob in 1994. Now check
http://en.wikipedia.org/wiki/AVN_Award and you'll see most actresses until 2000 didn't have boobjobs (at least at that time. Some never got'em, some got them some years later)
Take a look:
1994:
Shayla LaVeaux
Debi Diamond
Roxanne Blaze
Leena (not sure if she got a boobjob by that time)
Tianna
Porsche Lynn
Janine & Julia Ann (still with their original boobies)
all of them (with the possible exception of Leena) no fake boobage
1995:
Kylie Ireland
Asia Carrera
Ashlyn Gere (possibly by that time, not sure)
Tyffany Million (yes)
Kaitlyn Ashley (got them in 1997 or 98)
Celeste, Debi Diamond & Misty Rain (Celeste already had her big boobies)
1996:
Jenna (obviously)
Kaitlyn Ashley (got them in 1997 or 98)
Jeanna Fine (got them later, probably around 2000)
Ariana
Veronica Hart
Christy Canyon (naturals)
Felecia, Jenteal & Misty Rain (all natural by that time)
1997
Missy
Melissa Hill
Jeanna Fine
Shanan McCullough (naturals)
Juli Ashton
Janine
Melissa Hill & Jill Kelly (still natural)
1998
Johnni Black
Stephanie Swift
Dyanna Lauren (I think still natural, not sure)
Melissa Hill
Jeanna Fine
Silvia Saint
Missy & Jenna Jameson
as you can see, not very common until 99-00 or even later.
Anyway, I'm not saying it was because of that,
it seems to me it was one of the reasons, not the boobjob "per se", but the need for a perfect body, or put in another way, the absolute porn super star.
I remember people saying between 1996/00 Dyanna Lauren and/or Chloe would eventually replace her, but it didn't happen, they both lacked something. After that, it would be Brianna Banks (close but no cigar). I think nobody will get to the point Jenna got, because of many factors. I don't think the same conditions will ever be repeated (I mean the media state of the art context)
And btw, another thing is she filled the void left by Savannah, another blonde with perfect body who committed suicide in 1994 and was (arguably) the #1 by that time. The market was calling exactly for what Jenna brought, add to that the Internet explosion and you have it all. Jenna was (still is) for Internet what Traci Lords and Ginger Lynn were for VHS