Complex sentence: The main reason for Prince's version not charting is because it was a promotional airplay single and was not for sale; this was probably one the earliest examples of an airplay single, a kind of single that would prove very popular in the 1990s.
Split into simpler sentences: The main reason for Prince's version not charting, presumably, is that it was a promotional airplay single and not sold in stores. This was probably one the earliest examples of an airplay single, a kind of single that would prove very popular in the 1990s.