1) The decision to hire a consultant is (in most orgs) made at a level abstracted from day to day operations. So the person hiring you may simply not know what needs to be done.
2) The consultant's patron didn't take the time to get buy-in from all necessary stakeholders. This happens a lot at the VP level. You may have absolute commitment from your project's VP, but when you need resources from another part of the organization this turns into "what's that person's VP's relationship with my VP." Which may be great, or may be terrible. Navigating office politics to aid a project's success is 10-25% of being a successful consultant (result wise).
3) Inconvenience to ongoing work. I know if someone failed to loop me in and introduced a stranger to me on Tuesday, then asked me to do something that impacted my ability to do the work I already had scheduled, I wouldn't be overly happy.
4) Poor IAM (identity and access management) discipline and IT organizational structure. Consultant access and timelines are normally very different than new hires / "business as usual". The client may not be able (technically, process wise) to successfully complete an ad hoc request on an accelerated timescale.
If they cared, it would be handled or they wouldn't use consultants.
This is where providing transparency and insight (in a "how can we fix this?" way) up to management can help engagements go more smoothly. If everyone was aware from the first that "that QWSV system access issue" has been impacting the projects for weeks, that's much better than "the project is weeks behind" being communicated up at a random status meeting.
I wouldn't advise to participate in the blame game or to point fingers. You are not here to cause troubles and trouble is bad for repeat business.
P.S. Organizations are dysfunctional and the majority of consulting time is wasted. Lesson one of being a consultant is to live with it. :D
But if shit really hits the fan, at some point management will be asking "Who was supposed to do this?" And between that and the finger possibily being pointed at you... soft skills in handling politically tense situations well are useful.