Mike Mearls has gone on record saying bonus actions are one of his big regrets among rules from this edition. But two-weapon fighting has been gimped in every edition since 2ed, in overcompensation for when it used to be a bit too good. Part of "fixing" Beast Master was removing the bonus action requirements, but as you point out there's still quite a bit of overlap.
On the other hand, Hunter's Mark gives you an extra d6 on every attack, including offhand attacks, as opposed to many such abilities that only add to your first attack each turn. Two-weapon fighting lets you do good 'nova' turns, but Duelist is probably the more balanced fighting style for melee Rangers.