I'm afraid the reason behind it might not be as intriguing/interesting as you expect. The C64 version uses multi-colour text mode for the logo. Since Atari does not have this mode (at 320x192 it is B/W anyway), and does not support more than 128 user-defined tiles, I opted for a simpler logo.
To my knowledge Plus/4 does not have multi-colour text mode either (only in bitmap mode; please correct me if I'm wrong), so I just went with the Atari version and a simple colour gradient.