I think Markdown avoids angle brackets because angle brackets are frequently used for inline HTML. Sure, Markdown could look at the content of each pair of brackets and try to figure out whether it's a link or a valid HTML tag, but that would introduce a lot of complexity and it won't always work given the diversity of tags and URL schemes.

Since angle brackets are used for "automatic links" in markdown (http://daringfireball.net/projects/markdown/syntax#autolink), a markdown processor already has to look at the contents.

Markdown will let you mention a bare link, e.g. <http://google.com>, and will turn that in a hyperlink with the URL as the linking text.