All boils down to costs. Just cheaper to put on 14" wheels as OE, than 15" wheels.
That said - I'm pretty sure that the 6th gen Tacomas/Hi-Lux (1989-1995) came with 14" and the standard OE wheel (195/75-14). The 215/65-15 was an OE option at that time on both the 2WD and 4WD variants. With the 7th gen (1996-2004) - the 14" wheel was the OE wheel on the 2WD, the 4WD came with a 15" wheel OE - but the 2WD didnt even have an option for 15" wheels, still a popular upgrade for many.
A PLUS+1 wheel upgrade will not hurt the truck at all - there is not much change in CG - most of the wheel mass is below the trucks CG anyways. handling and stability will generally be better with the larger tire, since you have to increase the treadwidth slightly and allow a shallower sidewall to maintain the same overall tire diameter between the 14" and 15" wheels. Otherwise, you will throw the speedo off, if you install a tire too tall or too short (I believe that the 7th gen came with a 195/75R14 tire and the 6th gen optional was a 215/65R15 tire - their overall diameter is within 2% of each other, truck's speedo will not be able to tell them apart).