Race walking is in because it is part of a standard athletics programme, and athletics is in the Olympics.
It's not about it being dominated, it's about it being popular. Basketball is dominated by the US, but people will sit and watch them twat Tunisia or whatever, because people enjoy the sport. Also, the minor medals are well contested.
As for baseball being popular throughout the Americas. First, no it isn't; and second not a single south American country has ever qualified for the Olympics.
Nicaragua, Dominican Republic and Puerto Rico all qualified once and none really set the world alight.
Cuba, USA and Japan all finished within the top 4 every time they entered.
So yeah, it's not competitive enough for Gold or the minor medals so it's not in.
Rowing is a sport often considered a bit crap, because hardly any countries do it, but it has had 15 nations on the podium just this year, baseball had 6 ever, and one of them won in the year the US didn't participate.
Also it's totally, totally shit.