At its core, RSS is a broadcast technology. You make the feed available, and it is open for anyone to access. By doing so, you provide a much better broadcast user experience than email. No problems with spam filters, no problems confirming email addresses, and no problems with email list administration.
With email, you choose your audience for each missive. But with RSS, your audience chooses you. Because your audience chooses you, there are no natural waypoints where you can ask for information. And without information, no ready way to target your message for an individual.
Of course, it is possible to provide personalized feeds. But the current infrastructure is going to fight you. How will you support reader referrals when the easiest method is sharing the feed url? And how will you support passwords without wide-spread support from aggregators? And most importantly, how will the personalized RSS user experience be better than email?
PS There is a special exemption for entities using RSS as their primary broadcast communication mechanism - a common vehicle to a common location may provide a better user experience than splitting messages across RSS and email.
Part of a series inspired by my contribution to Unleash the Marketing & Publishing Power of RSS.