Flexor- it's actually a quite simple answer:
Every magazine has themes. Sports Illustrated has sports, Comuter Gaming World has computers, and Dragon has d20.
However, d20 is a huge theme to cover.
So they split it into small, monthly themes instead.
Drow here (although I think they should have covered the whole underdark instead), Fighters here, Gnomes here.
Also, it gives the editors something to base the issue around, thus making it easier to decide on what articles are needed and such.
Imagine being a magazine editor, and receiving the following report:
"This issue is another d20 issue. Put stuff in it, but don't make it too bland. Make sure it's interesting, but not too focused."
Now imagine this report:
"This issue is a Fighters issue. Include articles about weapons, fighting prestige classes, and fighting spells. But include other articles to attract a broader basis of readers."
Also, it grabs the attention of new readers. A new reader could see "d20 issue" and say "meh..." or they could see a "Dragons! Dragonslayers! Dragonslaying Prestige Classes!" and say "Woah..."
At least, that's my take on it.