In a decades-old franchise like Halo, it’s unsurprising that there are elements introduced in one game that aren't brought back for the sequels. Sadly, between development time, fan backlash, and game balance, there are a number of reasons developers might elect to leave some features, characters and vehicles in the past.
Vehicles introduced early in the franchise have been staples ever since; the UNSC's boxy quad-tread Scorpion Tank and the Covenant's sleek and nimble Ghost have appeared in every Halo title to date; as of course has the franchise's most iconic vehicle, the multi-role Warthog. Thanks to the clear division of roles, and sheer nostalgia, these vehicles have firmly cemented their position in the Halo franchise.
But through half a dozen mainline games, several spinoff games, and an expanded universe spanning comics, animated shorts, novels, and tabletop wargames, a whole lot of new vehicles have been introduced, and it only makes sense that not all of them could hang around forever.
Whether due to a forgettable design, a lack of a clear niche or purpose, or just perhaps the whims of fate itself, many vehicles just didn't make the cut.