OK, I found the answer as to why the I-751 was denied. The officer was acting strictly by the USCIS guidelines as stated below. Note the highlighted areas. I would check the options in the I-751 denial, then IMMEDIATELY appeal that you had a good cause for spouse not attending. Then, hopefully, USCIS will re-schedule.
Chapter 3 - Petition to Remove Conditions on Residence | USCIS
G. Interview
CPRs who file a Form I-751 must appear for an interview at a USCIS field office, unless USCIS waives the interview requirement.[42] USCIS officers may consider waiving the interview in cases where:
The officer considers they can make a decision on the petition based on the record because the record contains sufficient evidence about the bona fides of the marriage and that the marriage was not entered into for the purpose of evading the immigration laws of the United States;
There is sufficient evidence in the record of the CPR’s eligibility for waiver of the joint filing requirement, if applicable;
There is no indication of fraud or misrepresentation on the Form I-751, in the supporting documentation, or elsewhere in the record;
There are no complex facts or issues that require an interview or sworn statement to resolve questions or concerns; and
There are no criminal bars rendering the CPR removable.
When determining whether to waive an interview, the considerations listed above apply regardless of whether the Form I-751 is filed as a joint petition, individual filing request, or a waiver. For a joint petition, the statute requires USCIS to interview both the CPR and petitioning spouse.[43] If the CPR is filing an individual filing request or waiver, only the CPR must appear for the interview.[44]
If the required party or parties fail to appear for the interview, USCIS denies the Form I-751, terminates the CPR’s status, and initiates removal proceedings, unless the CPR establishes good cause for the failure to appear and USCIS reschedules the interview.[45] USCIS determines whether there is good cause on a case-by-case basis.