The classic problem: cycles. Either you make the programmer manually deal with cycles via weak references, which can be error prone and allow memory bugs to creep in, or you have a secondary GC to collect cycles. The first one increases programmer complexity and the second one increases implementation complexity, which are, respectively, the first and second things Go attempts to avoid.

There's also the issue that ARC vs. GC is a tradeoff between CPU overhead and memory overhead, and that in the scenarios Go was initially designed for (Google's servers) memory is much cheaper.