Maps are intrinsically vector data, and a raster graphics display back then would have been low-res, 320x240 at most, making the map (and text!) really difficult to read. And then you’d need the rasterizer itself, using precious CPU cycles and memory bandwidth to turn perfect mathematical line segments into crude pixelated approximations. And yes, the memory needed for the framebuffer was also likely an issue. The question is more, why would they ever have used a raster monitor? None of the advantages of raster were applicable, and the disadvantages were all relevant in their use case. The 100% obvious choice was vector.