Wifi is a half duplex contended media. You contend with other users and other signals (bluetooth, microwaves, cordless phones). Plus you have to deal with the varying qualities of wifi chipsets (consumer vs enterprise). A handheld device such as a droid has a fairly mediocre wifi chipset in terms of signal, performance and power.
Then you're dealing with the 3G network. Once again contending for airtime. Plus 3G networks tend to do funny things with the way they handle packets, much different to your regular wired networks.
