I think that SDRs are making it easier for listeners to check some of the less traveled parts of HF. I run (and record) my SDR from 6800-7000 each night, so I will pick up pirates that are not on 6925 but are still somewhere in the 43 meter band. 6880, for example, has been active lately. In theory you can record even larger portions of HF, but the disk storage requirements go up proportionally. Plus the time to go through the recordings.
My memory is that back in the day (yes, I have a Radio Clandestine QSL, I think from the 41 meter band) pirates were more concerned about the FCC, and tended to be more random with their frequency selection. Always using the same frequency as everyone else was not a good idea. The upside to this (even today) is that you get some people tuning in who otherwise would not hear your transmission.
Back in the 80s and 90s I recall the lower part of 3 MHz band being used on a regular basis, and of course both above and below the 40 meter ham band, plus just above 15 MHz. And MW was a lot busier back then as well.
For whatever reason, that isn't the case today, and most activity is confined to the 43 meter band. And just a few frequencies, really. My guesses would be:
1. It maximizes the number of listeners. Just use 6925, and you're almost certain to be heard by someone.
2. The reduced fear of FCC action.
3. Propagation wise, it's a pretty good choice. You get out a few hundred miles in the daytime, and at night a few thousand miles (although like a donut with a skip zone around the transmitter location). Noise levels are not too bad, except perhaps in the summer if there's storms around.
If you want to compromise on one band, it's really the best way to go.