In our case, history mostly. The application we are using with it was originally developed (not by me) in the late 1980s in Borland Pascal and, I think, dBase. At some point in the early 1990s the original developers decided that dBase wasn’t good enough and because Borland owned Interbase at that time and supported it with their coding tools, they decided to move to Interbase (later when Firebird was forked from Interbase they switched over to Firebird to avoid potentially needing to pay licensing fees on the numerous client computers).
The application, hugely expanded compared to what it was in the early days, has a great deal of Firebird-related code in it now, so moving off of it would be expensive and not really provide any return on the investment.
In our case, history mostly. The application we are using with it was originally developed (not by me) in the late 1980s in Borland Pascal and, I think, dBase. At some point in the early 1990s the original developers decided that dBase wasn’t good enough and because Borland owned Interbase at that time and supported it with their coding tools, they decided to move to Interbase (later when Firebird was forked from Interbase they switched over to Firebird to avoid potentially needing to pay licensing fees on the numerous client computers).
The application, hugely expanded compared to what it was in the early days, has a great deal of Firebird-related code in it now, so moving off of it would be expensive and not really provide any return on the investment.