Looking at a decompiled data dump of Pokémon Emerald, each of Castform's forms has a subfolder in Castform's graphics folder. In each subfolder, there's both a normal.pal and a shiny.pal file, which list the color palettes for the game to use. These files are identical to each other for each of the forms except the base form, which points to this being a deliberate choice rather than a technical restraint.
Obviously we have no idea what the thought process behind this decision was, so technically this question cannot be answered. But I thought that being able to have this confirmation that it was, indeed, a deliberate choice was interesting enough to warrant an answer.
Sources:
Full decomp (also linked above)
Castform graphics folder. Each form has a subfolder with a normal.pal and shiny.pal file, which are identical to each other for each form except the base.
