They needed the money.
Sometimes it seemed the Russian space program would do almost anything
to raise money.....
(the book goes on to describe renting out the TsUP's lobby to a Czech lighting-fixture company, flying a Japanese journalist to Mir for cash, charging fees for interviews with program management, etc)
The Russians were especially adept at working with Western
advertisers. Down in the TsUP's main control room, an auditorium
where Russian ground controllers hunched over four parallel rows of
consoles beneath an overhanging mezzanine for observers, a large
Hewlett-Packard advertisement sat beneath the main viewing screen....
(the book then describes the Pepsi ad deployed from Mir and the abortive attempt to film a romantic comedy aboard Mir)
As 1997 dawned, the program was barely alive. Funding snafus were
cancelling or delaying almost every major Russian space launch. Every
cosmonaut crew that made it to Mir could count on having its mission
extended; this way, the program saved money on rockets...
Source: The 1998 Bryan Burrough book Dragonfly, pp. 61-63
TsUP is a transliteration of the Russian acronym ЦУП which stands for Центр управления полётами
"Flight Control Center"