The most pressing reason is that people login from multiple computers: home PC, laptop, tablet, work PC, friend's PC, internet cafe, etc. While it is theoretically possible to have a system that syncs your certificates - either through a cloud service, or a physical device you carry - this is beyond the capability of most users.
Now, could someone create an authentication service that does this for you? That takes care of all the certificates under the hood? Well, that is exactly what Mozilla Persona does.
The second reason is that there are millions of websites that already use passwords. To change all these to use some other authentication scheme (whether it's OAuth, SRP, certificates, or something else) would be an enormous undertaking - and why would a website bother? What's the incentive for them?
Because of this, password managers are a far more practical technology to solve the authentication problem. Ok, they are not quite as secure as client certificates, but the differences are fairly minor. Systems like LastPass give you a single-sign-on interface, and work with (almost) any website.