It was cost,I remember the article in MotorTrend reviewing the '01 Runners and Toyota acknowledged that very few Runners actually go off road and it just wasn't a costworthy option.

Of course on the trucks they kept it since I'm sure alot of Tacoma buyers do actually off road and that is still a target crowd for Toyotas but also look at the trim and comfort quality of a Tacoma,not exactly in the same class as the pricier 4Runner by far.
4Runners have a different buyer since they have the comfort and ride that the Tacomas will never have and take a look at the members on this board which is primarily a 4th gen group and the off road forum isn't that busy and coupled with how many 4th gens I see with 20" rims it's obvious 4th gens are more likely to be seen street cruising than off roading pretty much like any SUV nowadays.
I don't see too many Tacomas with oversize rims on them.