The answer to this question can be found in Chapter 7. It can be found on p. 124 and on p. 129 in the paperback edition of the book.
On p. 124, Diamond tells us why wheat was easily domesticated. He says that wheat, in the wild, is already edible. It already grows in large patches that can yield usable amounts of grain. It is easy to plant and it pollinates itself. Finally, wheat is rather easy to store.
Strawberries do not share these characteristics. In the wild, they tend to be rather small berries. They do not pollinate themselves and you certainly cannot store strawberries for a long time. All of this makes them difficult to domesticate.
In addition, Diamond discusses the problems caused by birds on p. 129. He says that birds ate a lot of strawberries. This means that they planted the seeds that were contained in their feces all over the place. The result was that strawberries kept on being “the little berries that thrushes wanted, not the big berries that humans wanted."
For these reasons, it took much longer to domesticate strawberries than it did to domesticate wheat.