In Greek we have a Nominative, vocative, accusative, genetive and dative. In Latin we have all those and also an ablative. When you want to study why some cases have disapeared and others didn't, you must go back to an older stage, Indo-european language. Then we see that also in Latin some cases almost dissapeared, cause they had the same endings at one moment in time as and other case, so this other case took over the functions of the case and this case was no longer needed! In Latin there were also two other cases; instrumentalis and Locative. The instrumentalis was no longer needed at a certain moment, cause the ablative had taken over his functions. So also the locative dissapeared and was replaced mostly by prepositions. The locative indicated the place and the time mostly. Now we use prepositions (in urbe) or an other case (Romae, ...). But there are some forms that lasted like domi, ruri, humi etc. (these are old locatives!). You can see here old locatives with an ending in -i. Romae was spelled Roma-i in old times. So in fact forms like Romae are locatives and not genetives. But it's easier, when you learn Latin to say that it is a genetive and so all grammars do (except historical grammars of course). That way you have to remember less cases! The Romans did this for helping you!
<br /><br />When Latin felt appart into many Roman languages the cases we know, were mostly transformed in prepositions too! <br /><br />Referring to the ablative - dative question; <br /><br />You all know that the endings of the ablative are often the same as the endings of the dative, for exemple; <br />feminis - feminis (femina), avo - avo, avis -avis (avus), mari -mari (mare) and also the endings on - ibus are the same etc, etc.<br /><br />In Greek we have many iota subscripta. The fact that a iota was subscribed means that it was pronounced less. So in later Greek (Byzantine Greek) it dissapeared. <br />The forms of the dative in Greek corresponded better with the ablative than with the dative in Latin. That's why the Romans took over the Greek dative with the functions of an ablative (!!!) and that they let the dative - case open in the Greek declension!<br />It has a morphological explanation. Also the ablative and dative are in most cases connected when you look at the origins. <br /><br />But if you want to now more about that, you should read a historical grammar, like; Palmer, The Latin language
. or Ernout, Grammaire historique du Latin
. (Frensh).<br /><br />But remember in the Greek declension the Romans had an ablative and no dative! In Greek it's the opposite!<br />