Several states receive quite a bit of income by selling the exclusive rights to publish that state's caselaw. In addition, it is quite expensive to publish and maintain a web site, and money for such a project is not available in all states. Caselaw on the Web ideally would include decisions from at least the last 30 years, a sophistocated search engine for locating relevant law, case summaries, headnotes, etc. Providing all of these on a Web site would be very expensive. Given the above, it is impressive how much substantive law is already available on the Web.
Every state now has a Web site with some substantive law posted.
Unfortunately the quality, accessibility, and completeness vary considerably from state to state. Rarely will you see more than the last few years of a state's supreme court opinions on the Web. Often the opinions that you find will only be summaries. You will almost never see headnotes included. And though many sites allow you to search for a case by name, they often don't provide an index so that you can determine which cases are available.