Thanks for the hint.
I will confess that I left it as a hint because it is a while since I used that sort of export and I could not remember the exact details and did not have time to try it out at that moment!
On the other hand I find I remember things more easily in the future if I have at least partly uncovered the solution for myself rather than just following instructions so I tend to assume that others will retain the same benefit from such activity.
As I recall the .txt export uses the Clipboard Tab options which are probably set to Tab delimited output files unless you have changed them.
The rationale would be that .txt files are very likely to be associated with something like Notepad on all windows systems and to provide some other form of output with a .txt extension could be very confusing. Indeed many people might expect a 'normal' text structure output when using a .txt extension, whatever a 'normal' output is.
It is a bit obscure. I would be tempted to give it some other extension - .bar perhaps unless that clashes with some other application? - since .csv is so strongly associated with comma separated values. But that just a personal thing.