I don't know offhand, but I imagine it would be either due to the breeding rate still being exceeded by the rate of death - unlikely I suspect, considering how common it was and the length of time some collections kept the taxon - or due to collections simply losing interest in keeping the species.
In the timespan in question, the following collections held the species according to ZTL:
Wilhelma - held c.1975 until 1995 (bred 1978, 1981, 1982, 1983)
Budapest - held 2000 until 2006 ( no breeding listed)
Vogelpark Avifauna - held 1990 until 2002 (bred 1991, 1993, 1994)
Artis - held 1972 until 2003 ( regular breeding between 1972 and 1999)
Poznan Nowe - held 1991 until 2004 (bred 1992)
Basel - held 1969 until 1994 (bred 1990, 1993 at least)
Lotherton - held 1993 until 2007 (bred 1996, 2003)
Bristol - held 1999 until 2005 (no breeding listed)
Chester - held 1990 until 2003 (bred 1994, 1998)
Tilgate Nature Centre - held 1993 until 1997 (no breeding listed)
Banham - held 1990 until 2000 (bred 1993, 1994)
Paultons Park - held 1992 until ???? (no breeding listed)
Hayle - held 1990 until 1995 (bred 1990, 1991, 1992, 1993, 1994)
Harewood - held 1995 until 2001 (no breeding listed)
Gatwick - held 1996 until 1997 ( no breeding listed)
ZSL London - held 1956 until 2000 (regular breeding from 1958, final breedings in 1994, 1995, 1996, 1997)
Newquay - 1992 until 1997 (no breeding listed)
Paignton - 1990 until 1999 (bred 1993, 1994, 1995, 1996)
Twycross - 1992 until 2002 (no breeding listed)
Curraghs - 1994 until 2008 (no breeding)
Amazon World - 1997 until 2000 (no breeding listed)