I believe because Amiga was very old. Dreamcast is 15 years old and Amiga is 30. Also Dreamcast has more power, easier to do stuff with c++ than on amiga. Also there was no profit in the Amiga market anymore. 5,000 copies at 20-30 euro each, are still serious money to be made.
And I think the Amiga market was dead by 96 and c64 by 92.