He was playing to people whose ideas on race got frozen somewhere between Eddie Cantor’s “Mammie” and two fat white guys playing at being Amos and Andy on the radio. The progressive people weren’t in his demographic.
Remember, prior to rock and roll, kids and parents listened to the same music. Welk was probably as popular many bandleaders of his day – not up to the top, like Glen Miller or Tommy Dorsey, but successful enough. And teenagers in the late 40s listened to him much like their parents did.
Once rock became established, though, teenagers dropped the big band music and never went back (except for the brief swing revival in the 90s). By 1960 (and probably before, Welk was performing music for those who came of age prior to 1950.
YouTube links are essentially useless behind dialup. What is that link supposed to tell all of us?