Why can't we dig deeper?
Here is a thermal profile of the borehole:

(Source: Fig 5 in Yuri A. Popov, Sergei L. Pevzner, Vyacheslav P. Pimenov, Raisa A. Romushkevich,
New geothermal data from the Kola superdeep well SG-3,
Tectonophysics,
Volume 306, Issues 3–4,
1999,
Pages 345-366,
ISSN 0040-1951,
https://doi.org/10.1016/S0040-1951(99)00065-7.)



This borehole is (almost) vertical and not an angle, so its depth is true (11662 m) and it is quite warm down there, around 180 degrees Celsius as shown in the above figure. I assume temperatures were similar in other shafts drilled at maximum depth at 12262 m. 

The equipment they used initially for the 1982 borehole down to 11662 m was made to be able to work in thermal stability up to 200 degrees Celsius (Source: Kozlovsky, Yevgeny A., and N. I. Adrianov, eds. The superdeep well of the Kola Peninsula. Springer Berlin Heidelberg, 1987.) Consequently I assume the equipment used for the later shaft that reached 12262 m was of similar or better make than the original.

The problem is not the temperature against the barrel only: there is heat due to the shear (rotation) and let's not forget the weight of the shaft down the base. This is why the thermal stability of the system is to be considered in addition to the resistance of single parts of the drill system. So while hotter than they expected, it was within the operating range conform to the specifications of the make - but yet very near the thresholds and thus problematic.

The problem lied also with the rock being more and more plastic deep down in the borehole, due to pressure, porosity and water, making it a very difficult challenge to keep the borehole open each time the shaft need to be replaced or removed, adding difficulties to this already never-been-done-before project.

So this sum of challenges are probable reason why the project was interrupted before reaching the initial goal of 15 km.