It's a little more complicated than that. It seems like the narrative is now that the GG Bridge District conspired to keep BART off a lower deck of the bridge, driving the cost of the project up (San Mateo also left BARTD, shrinking the tax base for the district), leading BART to ask Marin to leave so that the backbone of the project could be completed.
Like with so many visionary transit projects, it only takes a few malcontents to make a terrible, insanely costly decision that negatively impacts millions in the future.
https://www.sfgate.com/local/article/Marin-County-BART-Golden-Gate-Bridge-study-14364699.php
