I can understand why the questioner asked this question , Murali played 13 less games than Shane Warne and still he managed to get 92 more wickets than him which is a huge number
Even though Murali clearly leads in the number of wickets, Warne is called ‘The King of Spin’ for some of the reasons given below:
Murali's performance against Zimbabwe and Bangladesh
Murali picked 89 wickets against Bangladesh and 87 wickets against Zimbabwe. These two countries together which add up to 176 out of the 800 wickets of Murali
Warne, has only 11 wickets against Bangladesh and 6 against Zimbabwe.
It was definitely Murali’s advantage that he played more against them. But obviously that wasn't his fault either.
Chucking Incident
I think the chucking accusation caused him to be less considered as King of Spin as Shane Warne. After bowling so many overs in Australia, suddenly umpire Daryll Hair called him for “throwing” continuously.
It was cleared by ICC and again umpire Emerson called him again for throwing when Murali started bowling leg spin against West Indies.
ICC cleared his action again and again. Many people still call him chucker which is quite unfair considering his action was clearly fair. Whereas Warne didn't had to go through such incidents. And therefore he had a cleaner image.
More competition for Warne
Warne had played in a fantastic Aussie team which contained McGrath, Gillespie, Lee and MacGill. Warne had to be the best among these bowlers. It was tough to pick wickets when you have such an excellent line up who all can win matches on will whereas Murali had only Vaas to with him, so he got chances to pick more wickets and lead the hurdle
Spin friendly conditions for Murali
Warne played most of his matches in Australia. This is one country in which many teams prefer playing without a mainstream spinner in their playing XI. Warne picked up 300 wickets in Australia which is huge considering the not so spin friendly conditions, it is an obvious fact that he grew up playing in these conditions, still, 300 is no small number in Australia
Most of Murali’s matches were played in Sri Lanka, India, and Pakistan. These are spinner paradises. And he has, 600 -odd wickets in these three countries, whereas he has less wickets outside Asia
Confidence level
Brian Lara told this in an Interview.
“Muttiah (Muralitharan) always confused me in the early part of our confrontation, whilst Shane Warne didn’t. Having said that, as my innings grew I felt I had more control over Muttiah, whilst Shane seemed to grow in confidence. They were definitely the two best spinners of my time,” Lara said.
Warne even after getting smashed for sixes and fours had the guts to flight the ball like anything hoping to get a wicket. While Muralitharan had lost some of confidence to fight the ball.
Ball of the century by Warne
In the 1993, Warne plays his first Ashes Test at the Old Trafford, Manchester. Then followed a memory that was etched in all cricket fans’ hearts. Shane Warne produced a ball that pitched outside leg stump to hit top of off stump to English batsman Mike Gatting. He had produced the ‘ball of the century’.
From this ball of the century, everybody called Warne as the King of Spin because he had produced a gem of a delivery which pitched outside leg stump and hit the top of off to make a fool out of Gatting.
Due to all of the above reasons Shane Warne is called the “King of Spin” And not Murali though both were legends. And personally I like Warne more as well.