I've stated my opinion a couple time already, so I'm not sure what else I can offer at this point. I believe Fournette was signed as an insurance policy after Damien Harris went down, then the Bills rode the hot hand with Cook & Ty Johnson contributing. Murray & Lenny may be a similar player so the Bills decided to stick with the status quo.
Fournette potentially being out of shape may be an easy guess, but it's not my opinion. I've scoured the Internet as to why Lenny may have not been activated and I haven't been able to find anything, so I'm not going to spend time on pointless speculation.
I'm hoping with Johnson being questionable as of yesterday's injury report, Lenny will be active Saturday. This would make sense to me.