It's more complicated than that. He was acquitted way more often than he was convicted. Clearly various judges along the way had some sympathy for his position, and/or for the patients, and/or found the law against assisted suicide unconstituional. He provided assisted suicide for more than 100 people. He was only found guilty for the final one, and even that was NOT for assisted suicide. That charge was dropped. He was convicted of second-degree murder and delivery of a controlled substance.
Interestingly, he promised to starve himself to death if sent to jail. Clearly he changed his mind about that.
Here's the timeline up to 1999, if you're interested:
Chronology | The Kevorkian Verdict | FRONTLINE | PBS
(Slips, this was in reponse to the idea that jury didn't even consider the wishes of the patient.)