The reason why the conversions on Megadrive/Genesis were so good is because Sega handled them for themselves.
Tiertex, Teque, Twilight were responsible for converting most arcade titles.
Strider is CAPCOM by the way
But your point is taken, most arcade conversions on Amiga were shite, and the only reasons are money and time.
Most games were done for the Atari ST first, as at the time, it sold more game units than Amiga. The Amiga version would differ in that it would use the blitter and copper and maybe they might jazz the music up a little, but they would essentially be identical.
The reason why it was rare for the Amiga version to be significantly better (althougth Double Dragon 2 on Amiga was better than the ST version because Richard Aplin wrote that version first) is because if you allow yourself to convert an arcade machine and make it as best as you possibly can for Amiga, you then make it so much harder to convert down to Atari ST. Sure, the graphics would look nicer in 32 colours, but, they would then look horrendous in 16 on the Atari ST, thus creating more work just to make it look ok.
Hybris, Battle Squadron, Team 17, Psygnosis all showed what could be done with Amiga specific games, with no compromises. Has anyone actually seen Shadow of the Beast on the Atari ST?
US Gold were a bang them out quick operation to get in the cash, so long as the game was vaguely reminiscent of the arcade in question, thats all they gave a toss about.