Arguably the reason Kerberos isn't used over the public Internet doesn't have to do with the security of the protocol, or the exposure of the KDC, but rather that it's an authentication model that doesn't fit the needs of most "public Internet" applications.
To quote Wikipedia, Kerberos "provides mutual authentication — both the user and the server verify each other's identity." That means that the client machine needs to have the necessary keys to vouch for its identity before user authentication takes place. Distributing those keys for "public Internet" applications isn't practical for several reasons - consider how many PCs access your banking web site, owned by a wide variety of people whose ability to install Windows patches regularly is strained. It's not like corporate IT will come around and configure Kerberos for them.
Kerberos requires tightly synchronized clocks - not that hard to do in a centrally managed corporate or educational environment, a lot harder when the machines are all owned and managed by completely different people.
One of the advantages of Kerberos is the ability to seamlessly leverage an initial authentication into multiple application accesses. On the public Internet, the multiple applications rarely have anything to do with each other - my bank, my mail, and my /. account have no call to trust each other or the same set of people. (This falls into the area of Federated Identity, and there is work in this area, but it doesn't need the same mix of things Kerberos brings to the table).
In short, Kerberos is a heavyweight solution, and public Internet application access control is a lightweight problem.
To answer your actual question directly, no, I don't believe concerns about brute-force attacks or vulnerability of the KDC keeps Kerberos off the Internet. The protocol has been reasonably well vetted, attacked, fixed, and updated over 20+ years. The client (machine) authentication piece alone provides tremendous protection against a number of attacks that are expected on a large open network like the Internet.