I'm not the author but I guess there would be a few reasons:
1. Familiarity with x86 assembly. It's probably the assembly language that the majority of people who write assembly language know. 6502 probably being 2nd place
2. Easier to port small x86 assembly programs
3. Even if it's stuck in the 80's it's the most widely used assembly language and there are dedicated online communities of people still writing x86 asm today in 2011.
1. Familiarity with x86 assembly. It's probably the assembly language that the majority of people who write assembly language know. 6502 probably being 2nd place
2. Easier to port small x86 assembly programs
3. Even if it's stuck in the 80's it's the most widely used assembly language and there are dedicated online communities of people still writing x86 asm today in 2011.