Why is the GPL incompatible? Just curious on the legaleze.
The BSD license is a very permissive license. You can do lots of things with the software, including taking the software and creating something like FreeNAS, and then not giving away the source code to the product that you're trying to sell to make a living. This is why Apple OS X userland is based on FreeBSD. Microsoft has used BSD code in their TCP/IP stack. The licensing primarily requires attribution, once you meet the terms of the license, it's available for use.
The GPL is ... different. Proponents like to refer to it as a "free" license, but it is an unusual definition of "freedom" that imposes significant restrictions on what you can and cannot do, which, to my mind, is not "freedom." The GPL is an attempt to coerce the people who benefit from software into providing their code back to the community for free. This seems like a nice idea on one hand, but if we were to look at a product like TrueNAS, it means that there'd be very little to protect a company such as iXsystems who has spent a huge amount developing a quality product from the nefarious types who might release GrinchNAS and sell it for cheaper, because, well, y'know, copying code is tons cheaper than developing it.
For more details, see
https://www.freebsd.org/doc/en/articles/bsdl-gpl/article.html
For a very quick TL;DR ...
[Y]ou had copyright, which is what the big companies use to lock everything up; you had copyleft, which is free software's way of making sure they can't lock it up; and then
Berkeley had what we called ‘copycenter’, which is ‘take it down to the copy center and make as many copies as you want.’
— Kirk McKusick, BSDCon 1999