Why doesn’t the fourth generation iPod touch have a wireless window?
As I'm not part of the engineering team I can't tell you why they've resited it, but in the iPod Touch 4 they'd moved the antenna toward the front of the display, rather than toward the back as they had with the previous generations. I'd imagine they've tried it in both locations but due to the way they've slimmed it and moved other components around they probably found that it was a better trade off to have the window back in place.

Step 19 shows how it'd been moved for the iPod Touch 4. I imagine as soon as iFixit has the new one we'll see some more information about why the window is back in relation to the other components.