a) "the magnetic interlocks have been ruptured, plasma coolant is leaking and a warp core breach is imminent. Riker orders Troi to evacuate the engineering section at once and Data to prepare the ship for an emergency saucer separation."
Whatever the "magnetic interlocks" really do, Memory Alpha doesn't say. It's possible that they prevent the core from being ejected, or the core itself became stuck and could not be ejected quickly enough. Or once the plasma coolant begins to leak, the controls to eject it are in the vicinity of the leak, and nowhere else, making it impossible to quickly eject the core without being overcome with coolant exposure. Either way, ejecting the core was never presented as an option. It was "we have a problem / evacuate / separate / get away / ka-boom / crash."
b) Time. LaForge believed he had five minutes before the warp core would breach. He did not. There was very little time for all personnel to evacuate from the secondary hull into the saucer. At that point, separating the saucer became the highest priority, and until separation was complete, it wouldn't matter if the ship was stationary or trying to get away from Veridian III as fast as possible; the star drive was coming with the saucer. Right after saucer separation was complete, the warp core blew earlier than was expected. The shock wave expands outward in all directions spherically regardless of direction, but even more critical, helm control was disabled, which prevented the bridge from steering the saucer into orbit. Had they known that helm control would be knocked out, they may have tried to get into a higher orbit to begin with. Instead, they became a falling brick, and all Data could do was reroute auxiliary to level their descent.
Because Paramount wanted a new Enterprise. That's why. If ejecting the core could have saved the ship, it was off the table.
Wow, Wojo, what an insightful and well-thought-out response! Thank you so much for gracing the message board with your wisdom!
Aww, thanks, Wojo, you're too kind. It was the least I could do.