straight slots in their sockets. Sometime in 1915, an experimental bayonet designed by E.K. Kabakov was issued in small numbers. This was no more than the blade of a Berdan II bayonet welded to a simplified socket. It was not manufactured in great numbers, however, and suffered from being 44g heavier than the standard M1891 bayonet. While this does not seem a massive increase in weight, it would alter the bullet’s mean point of impact while shooting with the bayonet fixed. When the Soviets modernized the Mosin-Nagant rifle in the 1930s, the rotating locking ring was replaced by a spring-loaded button catch with a chequered head to make the bayonet easier to fix or detach.
Bill Harriman: "The Mosin-Nagant Rifle", Bloomsbury: Oxford, New York, 2016.

The changes of doctrine explainedfrom here:

B.W. Menning: "Bayonets Before Bullets: The Imperial Russian Army, 1861-1914", Indiana University Press: Bloomington, 1992.

To illustrate the difference in contemporary use:

 
Note that both soldiers are from different branches of the German imperial army. It's just about the bayonets –– and the reach argument respectively the form.
(Klaus Lübbe: "German Uniforms And Bayonets 1841-1945: 676 Pictures Of Soldiers With Their Bayonets", Niemann: Hamburg, 1999.)

Finally, the most important reason is that it is with this exact configuration, with the bayonet attached, that this rifle was broken in. Removing the bayonet would decrease accuracy.

On the other hand, this rifle had some serious drawbacks. First of all, in all patterns prior to M1938 and except for the Cossack rifles, all rifles were intended to be carried and shot only with the