WMVP is the only station that puts an M-3 predicted 25 mV/m signal over all of the City of Chicago. WLS is weak on the North end of Chicago. It was even 50 or more years ago. It's surprising that WCFL didn't beat WLS in the ratings often in the late 1960s and early 1970s. Format, audio, DJs all about the same quality.
Well first, how "weak" could WLS's 50,000-watt signal been in the north end of Chicago? In the 60's and 70's, before a lot of computer issues and new development, the signal had to be very competitive.
Second, you are dismissing the programming elements as not having been a factor. I was an observer from afar, listening to both stations at night in Baltimore. My perception is that when WCFL flipped to Top 40, WLS had grown complacent and lazy. So at that point, while I didn't feel WCFL was great, they had a "more motivated" sound than WLS. Then in the very late 60's, when WLS revamped, they were far better than WCFL. The early 70's, when it was Super CFL vs The Rock of Chicago, I thought both stations sounded incredible.