The Joel concert occurred in 1987 as part of Gorbachev's 1986 Glasnost/Perestroika policy, which helped mark the beginning of the end of the Soviet Union due to suddenly opening the society to western influences without state control.

Prior to Perestroika, you could be imprisoned or harassed by state police for holding concerts of any kind that were not specifically sanctioned by the state. Russian rock music existed before then, of course, but the Russian state apparatus would alternately harass, ignore, or attempt to co-opt these movements, in events like the 1980 Tbilisi Rock Festival, a state-sponsored music contest that featured numerous amateur acts being judged by state-approved musicians (and to which some known Russian rock acts were not invited). From the beginning of Perestroika, numerous bands with a western sound suddenly had license to operate openly. As late as 1984, they were still being suppressed:

http://rbth.com/arts/2013/08/24/how_soviet_underground_music_rocked_perestroika_29179.html

By 1991 the situation was completely out of control of the Russian state apparatus. The Metallica concert was held only four months before the coup attempt by Russian conservatives, which served as their last-ditch attempt to revert back to centralized state control. It's important to note also that several other big American acts were on the bill (AC/DC, the Black Crowes, Pantera), that this was not the first major western rock/metal concert in Russia (Ozzy Osbourne, Bon Jovi and Motley Crue held a two-day event in 1989, for example), and that the 1.6 million figure was probably pulled out of the ether, as initial estimates were 150,000-450,000. Ozzy's show pulled in a similarly enormous crowd: https://www.youtube.com/watch?v=bC4ivKoPqsI

Source: http://www.nytimes.com/1991/09/29/world/heavy-metal-groups-shake-moscow.html

All this being the case, it doesn't seem like the Metallica show was uniquely significant, aside from the fact that it was a free show at an airfield. But by this point you had numbers of people showing up to these shows that were beyond the ability of police to possibly control or monitor. It's a difference of six years of liberalization with Metallica, after decades of suppression.