Primarily cost. The P3 is a good chip. However, as of right now a P3@1 gig will run you $199.00. With the same amount of money, you can purchase a 1.33 gig athlon for ( $197.00). The athlon chip will give you much higher memory bandwith, and has a stronger FPU. Also, if you get a 1 gig pentium 3 right now, this is the highest you will be able to go unless you plan on purchasing a new motherboard as well. While it is hard to say at the moment it looks like the athlon platfrom will go to 2.2 gig.
A little bit of knowledge is a dangerous thing!