Why no SQL Database Backend?

[blfs at comcast.net]

I have been puzzling over why GNUCASH was not written on top of an existing database engine. 

There are several excellent GNU databases out there. 

The natural design of a program such as GNUCASH would be the something like SQL-Ledger . 

This would solve all kinds of problems with this program. I understand that the program is not 
multi-user. If this program had been developed on top of existing excellent SQL DB this issue would
be easily solved. 

Also for any kind of real use of this program in an endeavor of any size the user would
need to be able to run SQL programs on the data.