There were a couple of things that the Wahhabis objected to when it came to the practice of Islam in 18th century Arabia. The most important reason was idol worship. The Wahhabis thought people practiced their religion the wrong way - according to Kur'an and Islam, idol worship is forbidden, which people obviously weren't aware of. Additionally, smoking and other vices were banned by the Wahhabis in order to restore the 'true' Islam.