Which variant of the BSD license is it? I don't see any license terms in the repo you linked to.

Why is the original BSD license incompatible with the GPL?

Because it imposes a specific requirement that is not in the GPL; namely, the requirement on advertisements of the program. Section 6 of GPLv2 states:

You may not impose any further restrictions on the recipients' exercise of the rights granted herein.

GPLv3 says something similar in section 10. The advertising clause provides just such a further restriction, and thus is GPL-incompatible.

The revised BSD license does not have the advertising clause, which eliminates the problem.

From here, see also What does it mean to say that two licenses are “compatible”? and License Compatibility and Relicensing.

In reply to Re: Licensing clarification: Requiring a BSD licensed dist in a Perl licensed dist by haukex
in thread Licensing clarification: Requiring a BSD licensed dist in a Perl licensed dist by stevieb

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