The bundle symbolic name for h2database is:
Bundle-SymbolicName: com.h2database

however the sources jar contains the following entries:

Manifest-Version: 1.0
Bundle-ManifestVersion: 2
Bundle-Name: H2 Database Engine Sources
Bundle-SymbolicName: org.h2.source
Bundle-Vendor: H2 Group
Bundle-Version: 1.4.200
Eclipse-SourceBundle: org.h2;version="1.4.200"

Since org.h2 doesn't match the main Bundle-SymbolicName the sources are not linked in Eclipse.

Looks like this is defined in https://github.com/h2database/h2database/blob/master/h2/src/installer/source-manifest.mf