It was a style of cabaret singing of which he was one of the extremes, with vibrato on any note held for more than a beat, notes held for much longer than a beat, and an especially delayed syncopation. It is easy to parody, but many people liked it, as evidenced by his 35 years playing at the prestigious Carlyle Hotel in New York. The Carlyle is a Depression-era Art Deco place, famous for its celebrities, such as Mick Jagger keeping a room there permanently for when he comes to New York, and JFK's sexual escapades, during which he learned the network of tunnels underneath the place.
No one seems to much like the style now, but like pronunciations of words or the width of ties and belts, it doesn't make it inferior. Sometimes unpopularity is a product of too much success, so that people get tired of your ubiquity, or decide they want to stand apart from the crowd by not liking what everyone else does. Ask the Bee Gees (well, there's only one left now) about how absolute domination of the market can suddenly work against you in a year, leaving you scrambling to stay afloat in music by working more behind the scenes, writing all the songs on the albums of Barbra Streisand or Kenny Rogers or quietly producing songs for a dozen others while waiting for your voices to become less radioactive.