The basic reason why is that classes in the "default" package (i.e. no package) are not visible to classes that ARE in a package.
This was a change back in Java 1.4 I think - to close a security loophole.
The Tomcat classes running your tag are in a package. Because your tag wasn't in a package, Tomcat couldn't find it - hence the error message "SelectTagHandler cannot be resolved to a type"