People did not care much about Stalin in 70s-80s. It is symbolic in this case (space ships in Russia are made from Titanium). Another space-related monument in Moscow (the Conquerers of Space) is also Titanium.
Also, from what I've heard, steel technology in the USSR was not that great and it might be just too expensive to build these from steel.
In that time period, the USSR produced most of the world's Titanium - a metal critical for defense and aerospace industries. This was the equivalent of, say, South Africa commissioning a statue covered with diamonds.
