Alexander II emancipated the serfs and was on his way to making a constitution when he died, while his son proceeded to return to complete autocracy what caused this rift in views between the two monarchs?
Alexander III certainly was much more reactionary and conservative than his father, the "Tsar Liberator" Alexander II. In this respect, Alexander III was very much like his grandfather Nicholas I in that both emperors staked a claim to a Great Russian identity that was against liberal ideals like constitutionalism. However, in terms of actual policies, the differences between father and son were not so stark.
The Great Reforms of Alexander I were not so much a transformation of Russian politics as they were the evolution of long-standing aspirations of would-be reformers in the Nicholavean period. The signature issue of the Great Reforms, the abolition of serfdom, had been something that had been percolating within the Russian government for decades before the Great Reforms. The vast numbers of serfs within the state had becomes something of an embarrassment for many educated Russians during the early nineteenth century. Russia's large-scale use of unfree labor appeared more backwards and less Western as the industrial age dawned. More importantly, there was an increasing sense among the Russian bureaucracy and state officialdom that serfdom was an inefficient system to harness Russia's reserves of manpower for the purposes of the state or the economy. The problem was not conceptualizing the problems of serfdom but rather getting rid of the system. Serfdom was a social system deeply intertwined with state institutions like the army. The serf issue was one of the third rails of Russian politics under Nicholas I and any official that broached the subject often got burned by it. It was only the defeats of the Crimean War that removed the various institutional blockages that prevented a reform of the system. The underperfomance of the army meant that the champions of reforming serfdom had a powerful backer in the form of the military. Serfdom had been integral to the ancien regime's form of conscription, but the Crimean War had made it clear that such a form of recruitment was not amenable for a modern military power.
The military issue underscores another aspect of the Great Reforms: they were more conservative in nature than they appear at first glance. Alexander II may have abolished serfdom, but he did so in a graduated manner and with an eye towards social control. Emancipated peasants suffered under a somewhat exploitative system of payments to their former landlords that created a kind of neo-serfdom in which peasants were free, but bound by debt instead of birth. Russian peasants may have been emancipated after the 1860s, but the state still controlled their movements. The Great Reforms introduced various forms of local governance such as the zemstva, but local control did not mean democratic governance. The zemstvo's leadership was drawn from the elite of the region. Peasants may have possessed an equal voice in zemstvo assemblies, but the procedures to get to the assembly still favored the nobility and the clergy. Even with this highly mediated form of local rule, the imperial state always looked askance at the institution. It is notable too that the zemstva were largely restricted to Great Russians. The imperial state did not trust ceding local control to non-Russians, especially Poles and Jews, and the system did not extend to sparsely-populated regions like Siberia.
Some components of the Great Reforms did manage to achieve a more open and liberal society. The restructuring of the Russian legal system remained one of the more signature aspects of the Great Reforms and it created one of the freer spaces in late imperial public sphere. Education also became much more open, especially higher education, both in terms of enrollment and the employment of teachers. Both the courts and the universities remained bastions for Russian liberalism through the reigns of Alexander III and Nicholas II. This liberalism though often predicated its politics not on challenging or confronting autocracy, but rather gradual reformism and squaring modernity with a tsarist state. But these aspects of the Great Reforms were more the exception than the rule.
Alexander III though had little personal stomach for reform and the geopolitical context did not push him in that direction. The assassination of his father reinforced both Alexander III's reactionary attitude as well as various grumblers over the Great Reforms within the bureaucracy. The assassination gave ammunition to those within the state who thought the reforms had gone too far. Alexander II's assassin was loosely affiliated with the populists By the same token, the assassination as well as the Polish Revolt of 1863 strengthened the chauvinistic tendencies of the Russian state. Geopolitically, the latter quarter of the nineteenth century was not the 1850s and 60s. The unification of Germany presented Alexander III a very powerful neighbor on Russia's borders. The speed of Prussia's victories in its wars against Austria and France emphasized the dangers of warfare in the industrial age. The authority of the tsar remained sacrosanct even during the most liberal period of the Great Reforms and there was now even less incentive for the Romanovs to divest or bound their political power. The Great Russian nationalism of Alexander III was not only the reflections of his own personal inclinations, but also was somewhat logical in light of the empire's position in the 1880s and 90s. The state was ill-inclined to view its subjects in the Western Provinces as loyal, which was problematic since this was the region most likely to become a battlefield if Russia became involved in a general European war.
Alexander III still carried forward a number of initiatives to strengthen the Russian state even with his somewhat reactionary politics. Sergei Witte's developmentalism had the sanction of Alexander III and the construction of vast railways and new factories took place under his reign. This was not as far removed from the Great Reforms since their rationale too was to strengthen the empire after the Crimean debacles. There is also evidence that Alexander II was less a liberal thinker and more of a realist when it came to the matter of reform. He may have toyed with the idea of a constitution, but he was no constitutionalist. Part of the reason why Karakozov assassinated Alexander II was there was hope that Alexander II's brother Grand Duke Konstantin would succeed him. The Grand Duke was much more liberal and open about the need for a constitution than Alexander II.
The assassination of Alexander II became one of the great "for want of a nail" events for generations of Russian liberals who projected liberal beliefs onto the assassinated tsar that he did not possess. The Romanovs were never inclined towards liberal solutions to modernity and there was a clear pattern that any step forward to a liberal direction was countered by a half-step back. Nicholas II only begrudgingly accepted the need for a constitution and the resulting document still granted the emperor broad and sweeping powers unregulated by the Duma. Had Alexander II dodged Karakozov bullet, it is highly likely that an Alexandrian constitution would have had a similar ethos.