My first thought of course would be the obvious one - that Fedor Tokarev fell out of favor with the Soviet regime. But all joking aside, why did the Soviets replace the Tokarev (7.62x25) with a less powerful Makarov (9x18)? I had wondered about this for some time and I came across this explanation in the book "Modern Combat Pistols" by Maxim Popenker.
The Red Army fought the Great Patriotic War of 1941-45 with both the semi-automatic Tokarev TT pistols and the obsolete Nagant M1895 revolvers. Despite the fact that more potent pistols were designed just before and during the war, the TT remained the mainstream weapon. Wartime experience, which included a close (and sometimes very personal) familiarity with German pistols, resulted in a major change of thinking about the role and necessary features of a military pistol for the Soviet army. Another factor that played a major role in the development of new requirements for the next military pistol, was the realistic prospect of a Third World War, with massive nuclear bombing and other such large-scale actions; as a result, pistols played a very minor role in both strategic and tactical doctrines of the Soviet Army. Furthermore, Tokarev pistols, despite being relatively simple and powerful, showed significant deficiencies, some of them quite serious, such as the lack of positive safety, so almost immediately after the war the GAU (Glavnoye Artillerijskoe Upravlenie – General Artillery Department of General Staff) issued a new set of requirements for a military and police pistol.
These requirements asked for a compact, double action pistol of the “Walther PP type”. New pistols were to be submitted in three calibres – 7.65x17SR Browning (proposed police round), 9x17 Browning, and a new 9x18. The last of these had been initially developed just prior to the war and refined after the war by the designer Syomin. Apparently, this round was inspired by the German 9x18 Ultra, which was designed in the mid-1930s to provide “acceptable maximum power” in simple, pocket-sized blowback pistols. The key reason for the increase in calibre when compared with the West 9mm rounds is unknown (the 9x18 Soviet has bullet diameter of 9.2mm, while most Western 9mm rounds have a bullet diameter of 9.02mm). However, with the benefit of hindsight, we can assume that the reasons for a calibre increase were probably the same as for the calibre of Soviet 82mm mortars, which were able to load and fire the slightly smaller German 8.1cm mortar bombs, but not vice versa. Also, while the Soviet Army was ahead of many others in the request for a double action pistol, it regressed somewhat in adopting an only marginally powerful round in a weapon that in essence was a pocket-type pistol. At the same period of time, many other armies, looking for an increase in power, starting to change their “weak” 7.65x17 Browning, 9x17 Browning or 7.65x20 Longue pistols to the more potent 9x19 Luger/Parabellum/NATO weapons. The explanation for this fact, however, is rather simple – while most Western countries relied on full-power rifles (bolt action or semi-automatic) and a sub-machine guns as a primary individual armament for the infantry, the new Soviet concept had no place for sub-machine guns, as the only primary arm of the infantry was the newly developed assault rifle. Most Western pistols were required to fire 9mm NATO ammunition just to have commonality in ammunition with the standard issue sub-machine guns; Soviet designers had no such requirements, and by the late forties 9x18 looked as if it was good enough for a military pistol.
Trials for a new pistol started in 1947. Many designs were submitted and tested, such as pistols by Baryshev, Rakov, Voevodin, Simonov, and Makarov. Some designs were submitted in only one of the desired calibres, some, such as the Makarov design, in two, and a few in all three. At the same time, the Army also tested few larger machine pistols in 9x18, which were intended as personal defence weapons for certain officers and NCOs. In 1948, the first trials resulted in a selection of the Makarov pistol in 9x18 as a next military sidearm for Soviet armed forces. However, it took three more years to refine its design, before it was officially adopted in 1951 as the “9mm Pistolet Makarova” or PM in short. In the same year Soviet Army also adopted the 20-shot, selective-fire Stechkin APS pistol in the same calibre. It was chosen over two similar machine pistols, designed by the then-unknown Kalashnikov (the designer of famous AK assault rifle) and Voevodin (who designed several pistols just before the war).
It must be noted that while TT was declared obsolete in 1951, it remained in service with the Soviet Army until the early seventies; in some rural departments of Soviet Militia (police) TT pistols served well into the eighties.
(from the book "Modern combat pistols")