I can't give you a definitive answer here, but I think the explanation is that the government of the day was swiftly moving away from emphasising government grants as being the sole method of providing funding to students and researchers, and towards a mix of methods of funding, and clearly had an interest in enabling the introduction of student loans.
The wording of the original act (http://www.legislation.gov.uk/ukpga/1988/40/part/II/chapter/II/crossheading/new-arrangements-for-funding-higher-education/enacted) is very clear in avoiding overuse of the word 'grant' and suggesting a mix of funding methods, and also emphasising that the staff of the new council are "to have experience of, and to have shown capacity in, industrial, commercial or financial matters or the practice of any profession".
Furthermore I would suspect that the new body was designed to improve the degree to which government power over higher education was not mitigated by an intermediary body. That's a suspicion based on the generally centralising role of the Thatcher governments towards any entities that wielded a degree of independent power. That said, the UGC had acted as a tool for the first round of cuts to higher education, so that point should not be overemphasised.
There's a good article here with a view of UGC from a university perspective in 1984: http://link.springer.com/article/10.1007%2FBF00128560#page-1
My recollection as someone who went to university in 1988 is that the writing seemed to be on the wall for student grants. And certainly by late 1989 there were major protests against the intention to introduce loans (http://www.onemanandhisblog.com/archives/2010/11/student_protests_1989_style.html). There seems good reason to believe that the abolition of the UGC was intended as a precursor to the introduction of student loans. Therefore the fact that the Student Loans Company would not be established until two years later in 1990 does not contradict the fact that this change was largely about the abolition of grants.
Edit to the above - as noted in the comments. The UGC 'grants' were block grants provided by government to universities, and not to be confused (as I did above) with student grants, which were paid by LEAs. Therefore the statement above that "that the abolition of the UGC was intended as a precursor to the introduction of student loans" is not correct.
I do suspect that the reforms were part of the doctrinaire increase of government power over a semi-autonomous body, and the move away from viewing the government as the sole-source provider of financing to universities via the block grants towards a view of universities seeking funding from public and private sources. However a fuller answer depends on somebody with more specialist knowledge than I have.