The reason is this. Google Reader updates feeds once an hour, less if there is only one subscriber (something that would be true for nearly all Google Alert feeds). As such, there will be an average delay of about 30 minutes before any new item appears. Email, in those cases, would likely be much faster, probably taking less than 10 minutes.

Granted, there are ways to mitigate this but what the speed issue comes down to is how you have your email setup vs. how you have your RSS set up. For some feeds will be faster, others will get more quickness out of email.

Still, I don't think that a delay of even an hour is going to be much to most Google Alert users. Most, I doubt, check their alerts often enough to notice.

Just my theory though, I could be wrong.

Thank you for your thoughts!