Redhat's summary of the technical reasons for the 69xx band being used are excellent. The only other points I would add are historical - pirate operations in the 80s often occurred in the 74xx range, just above the 41 meter shortwave band (and the 40 meter ham band as well of course). This lasted for quite a few years, until a variety of factors (mostly the 41 meter band expanding up into that region) pushed operations back down to below 40 meters, first on 6955 kHz. That was the hot spot for a while, until interference from a SWBC station (from Chile, I think?) caused enough QRM that operators moved down to 6925, where we are today. Plus of course all the adjacent frequencies being used, 6930 to 6970 in 5 kHz steps. 6850, 6875, and 6900 (plus or minus the crystal tolerance) also are fairly popular today. Between 6900 and 6920 seems to be a sort of a no mans land - probably avoided due to MARS operations on 6903 and 6913. Ironically, 68xx used to be somewhat popular back in the 80s, I have a QSL from Voice Of The Voyager from 6840.
And of course WRMI has now taken up residence 24/7 on 6915. Or should I say had? Checking my recordings for yesterday, I do not see them on 6915. Not sure if they have moved, or just really poor propagation. Anyway, I find/found them less of a nuisance than I thought I would as a listener, and more useful as a propagation beacon. Update: I just fired up the SDR this morning, and hear Bro Stair on 6855, so perhaps that is where WRMI moved to? Hmm. They just went QRT at 1406. Very strange. OK, back again.
Another reason activity has not moved far outside the 69xx band is that is where the listeners are looking. With so many running SDRs and waterfalls, we can spot signals anywhere in the 6800-7000 range as soon as they sign on, and log them. But unless the op lets us know, we generally completely miss transmissions on 10 or 13 MHz, or elsewhere. Several operators have mentioned to me that they have operated on 11 MHz many times, and not gotten a single report. There have been a few attempts to use 3 MHz (several months ago there were quite a few transmissions there), mostly to get around the low foF2 values at night, when 43 meters is completely closed for anyone within a thousand miles of the transmitter site. But in addition to people not checking there, you've got high noise levels (not as bad in the winter as summer), and I imagine antenna problems, both for the transmitter and receiver. Not everyone has an efficient antenna for 90 meters.
One other thing I will throw out - we are at what is actually a historic high period of SW pirate radio activity right now. Amazingly, unbelievably, high. This is probably causing a lot of the activity on frequencies other than 6925, since everyone can't transmit at once on the same frequency. I've often noted 3, and occasionally 4, 5, sometimes even 6 stations on at the same time. That was unimaginable in the 80s and 90s. If you want to talk about the "good old days of shortwave pirate radio", I would say they are right now. There is more to listen to now than at any other time. Ever.