Simple reason is they wanted an epic crash scene. but if you want the technical reason then i dont think that escape pods have impulse engines. only thrusters. (perhaps some techy knows for sure?)
so the saucer is moving away at full impulse and still gets hit by the blast. escape pods will be moving at thrusters only, (maybe a lower impulse at max) and even with the increased time still would have been closer to the ship than the saucer was so they would have been incinerated by the explosion.
if it has been a random explosion, it probably would have been fine, but this was a warp core explosion and the power/range would have been too much for the pods to escape.