You are very observant. The answer to your question has to do with differences in behavioral characteristics between deer and antelope.
Deer are "jumpers" and when they encounter a fence, adult deer will jump over the fence and attempt to cross the interstate to continue their journey/migration on the other side. Numbers of deer trying to cross the interstate will vary depending on the severity of the winter. "Hot-spots" for deer being hit on I-84 are between the east and west Twin Falls exits and the Tuttle to Hammet area. In both locations, the interstate borders historic mule deer wintering areas.
Antelope are "runners" and the only way for them to cross the interstate is for them to crawl under the fence or find an opening in the fence. Net-wire on the lower 3' of the interstate fences prevent antelope from going under the fence and they are unable to jump over the fence; so most antelope are content remaining on the north side of the interstate. There is a small population of antelope that have found gaps in the interstate fence system underneath the bridge that crosses the Snake River at King Hill. You will see these animals range all the way to Bliss in the winter. Fortunately, they have the memory capability to return to the fence gap in the spring to venture back to the north side of the interstate through the same openings.