There's a bit of detail in the wiki[1], you can probably get a better idea by searching the lists (there's likely a bunch on freebsd-current).

Normally when someone brings up git on the lists, the common response is simply, "there's already an official mirror[2]". SVN fits the FreeBSD development model -- everything is centralized.

Moreover, it can be argued that when the decision was made in 2008, DVCS's weren't as well-known as they are today. The relative maturity of SVN I believe was a large factor in the decision.

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[1] https://wiki.freebsd.org/VCSWhy

[2] https://github.com/freebsd/freebsd

On FOSDEM a couple of years back, that question came up during a talk as well. IIRC, FreeBSD needs a VCS where they can delete changesets permanently, for legal reasons.