It depends on public opinion. In some countries people think, taxes must be equal for all, or linearly grow (and with ideally smooth function).

They even sometimes change mind and make experiment with other function.

Many experts think, this is because in these countries, laws made entities tax agents (every entity pay all taxes for all hired people), so people just don't bother how many taxes they pay, and how much time spent to calculate all these things and to fill all papers.