Why does World of Goo on the iOS have to be downloaded via wifi?
I can't add much more to that...
On the app page it's got:
Must be downloaded with wifi connection
As opposed to using a 3G/4G connection. The reason is specified in the previous sentence: This is a big game...
You can of course download it on your iTunes on your computer first, then sync it to your iOS via the cable. The developers just didn't spell this out for ya (which they should have).
Note: I have downloaded much bigger games like Mass Effect: Infiltrator, which is more than 1G. You can install it via either Wi-Fi or iTunes on your PC/Mac. This is definitely not a game you want to download via 3G/4G.
Apple sets size cap for downloading apps by 3G. Last update from March 2012 make it possible to download app under 50MB.
World of Goo like other big iOS games are far far over this limit so they must be downloaded by WiFi or transfered from your iTunes.