Probably the two most important reasons are

1) There are usually no "legitimate" stations, such as broadcast,
military, ham, government, utility, etc., in the vicinity. Although
there haven't been any HF pirate busts in recent memory, having a
complaint about your station filed with the FCC by a legitimate station
has always been the quickest way to get caught.

2) Pirate radio DXers camp out on those frequencies, so it's pretty
likely that a pirate will be heard if he broadcasts there. This
eliminates the need to make any sort of announcement about time and
frequency when a pirate wants to go on the air. If the broadcast is on
an obscure frequency, there is a good chance that no one will hear it,
or, at least no one who is likely to post a log and ask for a QSL.

On 1/14/2010 9:24 PM, dave wrote:
OM wrote:
On Thu, 14 Jan 2010 06:01:12 -0500, dxAce
wrote:

WEAK Radio, 6925u, heard here at 1050.

...I've noticed that several of these pirate stations run either 6925
or 6930. What's the appeal for this frequency range?


OM
Herding instinct. Lack of imagination. Generally silliness.