It's not that. Sip has a good relationship with the coaching staff. Dirk has a very poor one. Sip gets better access and he usually puts out what the staff wants to be put out there, which isn't a bad thing. LJS has good, upbeat coverage. Other sources are more critical, and there's room for both.
What can you point to as evidence of this? Sipple has been critical of Pelini
often, and has taken heat from Stadium Drive for it, and from Pelini directly.
The difference between Sipple and Dirk is that Sipple is a professional journalist and understands how to maintain his professionalism while being critical. You're not going to see a public dressing-down of Sipple like we saw for Dirk, but you had better get your facts straight before making statements like the bolded above and "Sipple is the staff's mouthpiece." He is not, not even close, and he's borne the brunt of Bo's anger before.