Because the BSD license is a permissive/pushover license, which let you relicense other people's work as proprietary. The reason that copyleft licenses like the GPL are better is exactly that they don't allow this sort of behavior.
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99% of the code in Mapbox GL was written by Mapbox employees on company time. The GPL would have made no difference.
Except in practice, removing that 1% (or getting permission from its contributors) could be extremely complicated.