Albert Hourani, a British historian of Lebanese descent answers this question directly in his book "Arabic thought in the liberal age (1798 - 1939)":
"[P]olemics have their danger: in defending oneself, one may draw closer to one's adversary than one thinks.
It is significant that both his controversies were concerned, not with the truth or falsity of Islam, but with its being compatible with the supposed requirements of the modern mind; and in the process Abduh's view of Islam was itself affected by his view of what the modern mind needs. He carried farther a process we have already seen at work in the thought of Tahtawi, Khayr al-Din and al-Afghani: that of identifying certain traditional concepts of Islamic thought with the dominant ideas of modern Europe. In this line of thought:
maslaha gradually turns into utility,
shura into parliamentary democracy,
ijma into public opinion;
Islam itself becomes identical with civilization and activity, the norms of nineteenth-century social thought.
It was, of course, easy in this way to distort if not destroy the precise meaning of the Islamic concepts, to lose that which distinguished Islam from other religions and even from non-religious humanism.
It was perhaps this of which his conservative critics were uneasily aware: there was bound to be something arbitrary in the selection and the approximation. Once the traditional interpretation of Islam was abandoned and the way open to private judgement, it was difficult if not impossible to say what was in accordance with Islam and what was not.
Without intending it, Abduh was perhaps opening the door to the flooding of Islamic doctrine and law by all the innovations of the modern world. He had intended to build a wall against secularism, he had in fact provided an easy bridge by which it could capture one position after another.
It was not an accident that, as we shall see, one group of his disciples were later to carry his doctrines in the direction of complete secularism."