Why was X-Men Evolution cancelled?

Well, there were some rumours why this show was cancelled. Some claims that it wasn't cancelled but ended. However, the show producer Boyd Kirkland thought of cancelling the show because there was a conflict between management and financing partner regarding the budget for the show.

From Knows Why article,

Although many have claimed that the reason for the cancellation of X-Men Evolution was not really explained, many have speculated that the show was cancelled because the producers just felt the need of ending the show despite its high ratings. Others also claimed that the series was not cancelled but it was just the end of the series per se. In true light however, show producer Boyd Kirkland thought of cancelling X-Men Evolution because there was a conflict between the management and their financing partner regarding the finances involved in the production of the series. Ã’šÃ‚ With a monthly budget of $350,000, agreements did not turn out to work well that Marvel opted to cancel the show. Although the show was able to attract a lot of viewers, especially teenagers and adolescents, the predicament faced by the producers and the management led them to cancel the show. Even the publishing of the X-Men Evolution’s comic book was abruptly cancelled since its sales were found to be significantly low.