The tax is not based on the current value nor on the value 30 years ago. It is based on a value which goes up 2% every year. The taxes go up every year but in a predictable way, it is damping function on the wild swings of the bubble/crash/repeat housing market.

So it is very easy to see how prop 13 was passed. The real estate tax is one the major yearly budget items for most families. Having it vary wildly and unpredictably year to year is not something most families [who don't earn tech bubble salaries] can handle.