Don't forget they wanted to make the SE/30 a different kind of machine. There were no Apple laptops when it was introduced, so it was the small, all-in-one powerful machine. They probably feared a reduction in sales of the IIcx if the SE/30 could natively display color.
Using the same analog board as the SE (they are identical; I have swapped between SEs and SE/30s numerous times) also made it easier to produce (and, for SE owners, made upgrading easier, but using a single design on more than one computer is what Apple cared more about from a production standpoint). Having the same board also meant a lot of other components were the same (video board, cables to go to the logic board).