Because what he taught works. Buddhists are told to test things to check that they work:
[The Buddha said:] "Yes, Kalamas, it is proper that you have doubt, that you have perplexity, for a doubt has arisen in a matter which is doubtful. Now, look you Kalamas, do not be led by reports, or tradition, or hearsay. Be not led by the authority of religious texts, nor by mere logic or inference, nor by considering appearances, nor by the delight in speculative opinions, nor by seeming possibilities, nor by the idea 'this is our teacher'. Kalamas, when you yourselves know: 'These things are bad; these things are blameable; these things are censured by the wise; undertaken and observed, these things lead to harm and ill,' abandon them. ... Kalamas, when you yourselves know: 'These things are good; these things are not blameable; these things are praised by the wise; undertaken and observed, these things lead to benefit and happiness,' enter on and abide in them."
– Anguttara Nikaya, 3.65
It is for that reason that there are so many different Buddhist denominations; different techniques work for different people.