It may be because of lots of things and I won't be suggesting to check out bent/missing connector pins which you've probably checked out already...
I would check the following if I were you;
1) Does the fan turn? If not there may be either a major short on the 12V line OR simply the fan is dead. If the fan was dead for a long while (long before you got the board) the mobo must have suffered overheating and major components such as CPU or RAM ICs might have been fried.
2) If the fan turns, I would open the case to check the PCB and try to figure out what might be wrong. Major dust & dirt, track damage caused by major corrosion, blown-up tantalum capacitors (especially around sound section) may prevent it to run.
3) Did you try to fire it up without a B board? Normally without a B board you would expect to see a single colored screen (not white but blue, pink, green etc). At least this was the case for my CPS2 boards (Grey and green type) never had a US board.
If the PCb turns out to be extremely dirty (where you even cannot see the PCB itself - which is a common problem with CPS2 boards due to the plastic case and years of inhabitable
harsh environments) I'd recommend to give it a good bath (yes, under water with dish washing detergent and brush), let it dry and try again. If you notice severe track damage on the PCB or severely discolored/missing legs on some surface mount chips due to corrosion, I'd recommend just throw it in the spare parts bin. Really not worth the effort to fix it (been there done that - I've fixed lots of track damage on one board, 2 years later when I tried it again it failed on me again. The corrosion never stops. It continues to eat tracks "under" the solder mask)
Good luck...