The reason many adult standards stations are adding more newer cover versions of older songs is an attempt to introduce older music via cover to a younger listening demo while remaining true to its roots with the original standards in the overall mix each hour.
By way of example, Rod Stewart is a 70s and 80s rock artist who, at least ostensibly, brings "coolness" to the adult standards format creating the image that older music is still "hip" or hepicat. Kenny Rogers is another example of a former 60s rock and 70s, 80s and 90s country artist who has done an outstanding job contributing to the adult standards format singing older and even newer standards and ballads.
Without regularly updating the music to attract a younger demo, the format will eventually disappear as the current demo ages and dies off. This is one reason you no longer hear the Big Bands as a mainstream, or even niche, format on radio today.