It was easier to do the programming, not easier to use. On the ST you had 68000, graphics, audio, memory. On the Amiga you had shitloads of chips. If you have ever tried to do some assembly coding you should know what I mean. Also, programming to the ST=> more potential as you had the Amiga version almost ready. The opposite was not easy. Companies didn't care if a game was better on the ST or the Amiga. All they could see was two (one?) markets that could be beneficial to them. The ST users didn't have their computers long before the Amiga users had their Amigas. I don't believe that a few months changed anything....
A typical example of a game was Kick Off 2. It was written on the ST. The code was virtually the same (68000 assembly) but the Amiga chips allowed better manupulation of the graphics. Hence, the ST had no background for the field but the Amiga had. The code was the same though, only the graphics changed.