There are better routes from west coast to Chicago than there are from east coast to say, Denver. It's just the way it is.
People on West Coast get 40-50 ping to Chicago on average. If you're getting 70 ping then it's because your ISP is shitty.
I mean sure, it might have been better to move it to Kansas City or something to get the best possible balance, but I'm sure Riot had their reasoning.
I know for a fact that Chicago has pretty good internet infrastructure, so maybe it's the most "central" city that's least likely to have lag spikes from all the additional traffic. Would you rather have lower ping with EUW-style lag spikes or would you rather have slightly higher ping that's stable? I'd personally pick the stable higher ping every time.
Unless you're Challenger, 70 ping is going to have very little effect on your gameplay. Maybe a few things like ward hops are going to be a little harder, but if people from OCE could ward hop on 250 ping when we were on your servers, then you can cope with 70 ping.