Why Java 9 does not simply turn all JARs on the class path into automatic modules?

There are at least two reasons:

• Just as regular modules, automatic ones are suspect to certain examinations by the module system, e.g. not splitting packages. Since JARs on the class path can (and occasionally do) split packages, imposing that check on them would be backwards-incompatible and break a number of applications.
• The unnamed module can read all platform modules, whereas automatic modules can only read those that made it into the module graph. That means a JAR needing the java.desktop module (for example) will work from the class path but not from the module graph unless java.desktop also makes it into the graph (via a dependency or --add-modules).

I have no time right now to check the second but that's what the State of the Module system says:

After a module graph is resolved, therefore, an automatic module is made to read every other named module, whether automatic or explicit

Resolution works on the declared dependencies and an automatic modules declares none.