The reality is that the cost to build BART to Marin and Sonoma from SF would be on the same order as building HSRail from SJ to LA. Arguably, there's enough BA-LA traffic (with a couple of intermediate stops) to justify HSR; there's nothing in the North Bay that would justify such an expense.

After BART-on-the-bridge fell through on cost & engineering grounds, there was talk of a transbay tube from SF to Marin. Could have been done, at enormous expense, but the traffic just wasn't there. Also, of course, Marin took the same tack as San Mateo - we don't want no steenking trains - buses for the hoi polloi are acceptable, barely. BART, logically, concentrated on where the traffic and interest supported the project: SF and East Bay.