You can have a 800 MHz Pentium III and still get the same performance as a 33 MHz 80486 if you don't have a bus structure that can handle the higher performing processor. There are a lot of trade-offs in designing a computer. There are even more trade-offs if you need to have it operate in harsh environments. In short, lots of factors had to be considered, such as existing infrastructure, cost, how long the testing process will take, and what type of processing needs to get done. Giving all these factors the Intel-based 80486 was the best processor.
Space is very harsh.
On a side issue, better processor is relative.