I'd guess that because Jimmy hasn't the fighting skills to beat him yet in Chapter one ? Only one transistor on the roof of the Autoshop......Plus it's damn hard to beat him in the hole at the end of the chapter.....
Although, using the cheats, I have 'killed' Russell in the first chapter, prior to the hole fight, and it made no difference at all. So, again....What's the point in having the others inaccessable ?