Why optimus prime changed semi truck into old truck?

Optimus Prime transformed from a semi-truck to an older truck in the 2018 film "Bumblebee" to reflect his character's journey and the nostalgia associated with classic Transformers. This design choice also aligns with the film's 1980s setting, evoking a sense of retro aesthetics. Additionally, the older truck symbolizes a return to his roots and the simpler, more heroic qualities of the Transformers lore.