Here are a few good reasons why someone might choose not to make an RSS feed available.
(1) They want the intelligence that comes with crunching and reading server logs. In the early days of running my icann.blog, I took great interest in knowing how many unique visitors were stopping by, how often, and what they were reading most. With RSS, you know who is subscribing, but that's not the same as reading. My aggregators bring in a lot of articles, but when I'm pressed for time, I simply mark them as read without actually reading them. Serving a web page to anyone other than a bot though is usually a good sign that something on the page has been read.
(2) They might not want their content aggegated into a separate web page. For the reasons noted in (1), I was initially a little wary of allowing my articles to show up in the ICANN Blog Aggregator. At the end of the day, I reasoned it really didn't matter and having all of the icann-related web content show up in a single place was much more convenient. Others might go the other way though.
(3) They want advertising revenue from ad serves. With Google's AdWords and other services geared toward small publishers, you can now generate a modest revenue flow from a weblog or personal publishing site. Unless you can find a way to push the ad out via RSS -- and I think there's a good idea somethere in that thought -- you'd rather force people to the web to get the ad served than let them have the headline or content for free.