Because
Isabel is the spanish form of
Elizabeth or a semitic name that means "daughter of
BAAL" which makes a lot of sence because in spanish "hija de
Baal" sounds a lot like
Isabel (hijabaal).
As for Anabel... does not come from anything else but
Annabel which is the proper spelling of a name that is NOT Spanish. Even when you take it appart.
Ana is a name but Bel is not a sufix of any kind and Bel is not a name on its own. The word Bell standing on its own means nothing in spanish.
BUT.... If all of the above is wrong, then I would have to say that Anabel is a compound name that comes from
Ana and
Isabel, just as
Maribel is a compound name that comes from
Maria and
Isabel.