I just realized the number of discontinued anime will increase as time goes.
The anime may have served its purpose and there is no more need (to the production committee) to continue.
If the merchandise market has reached saturation for that product, that means those merchandise makers/vendors may either stop buying tie-in licenses or if they had a bigger role, they would drop out of the production committee which is the primary group paying the costs and making resources available to create the anime, distribute the anime, market the anime, and make it available to the public. At that point, there is no more need to continue the anime. A lot of times, the tie-in manga used to promote the anime and keep buzz going also dies leaving just the original light novel. Sometimes the author also abandons the LN in order to work on a new light novel that might hopefully get a manga and anime once again, thus giving him some royalty profits. The world is littered with LN that also stopped a year or so after the anime stopped as the author moved on to something else with the opportunity to get an anime so that he can earn some more royalties.