what blocks are 90% empty and there is a back log?
Essentially all. None of the last 100 blocks were larger than 400KB. Only 5 were larger than 300KB. The vast majority are 100KB to 250KB. More than 20 are <100KB. The block size limit is 1MB.
why are miners creating a back log when they have plenty of space.
why not include all the TX's if there is space for them?
Probably because far too many of those transactions are being sent with low (or no) transaction fees, and the miners just don't feel like turning charity into an entitlement.
I'd like to know the stats as far as the trends in fees go. But if that is the case, I can't say I blame the miners for not wanting to encourage the practice.