LotR was the more epic Tale, and as such more worthy of being filmed... in theory had the events occurred closer together or had more shared characters they could/would have filmed a Prologue telling Bilbo's tale... but at Three 2.5 Hour Epics as it was. they stuck to the LotR adventures...
then upon seeing the revenue and Interest after several years they decided to revisit the prequel/prologue and padded it out into it's own Trilogy of overly long movies as a shameless cash grab.