Over 99% of all living species that have been on this planet since the beginning of life have become extinct. This will be our fate eventually, unless God steps in.
The Thylacines seemed to have had a lifestyle similar to coyotes, but must have had a brain the size of an opossum without the wit to avoid extermination.
I think it would be unfair to say that the Thylacine's so called lack of intelligence caused its extinction. They were said to be a fairly intelligent predator, BUT were outcompeted by the Dingo, when it was introduced. Add to that, the negative impact from settlers. Australian/New Zealand ecosystem's are very delicate, with many species becoming extinct over a short period of time.