Reason for Jovian Planets
Because you cannot condense O by
itself (but only in compounds also
containing Si, Mg, Fe), you don’t
have much material available for
making terrestrial planets.  You are
limited by the low abundance of Si,
Mg, Fe:  Terrestrial planets are
relatively small
Once solid H2O becomes available
you have lots more material
Starting at Jupiter you can make a
big enough core from solid H2O that
you can gravitationally hold onto the
H and He gas