An issue with a knee problem arose during Tuesday's medical, resulting in the Toffees returning to the Dutch side with different terms.
Elstone told Sky Sports News: "(It's) very sad. We were very excited about the prospect of Leroy Fer coming to Goodison but the transfer didn't go smoothly in the end, there were issues.
"We have to put the club first. Our track record shows we do that pretty well. We thought long and hard about it and it didn't work out."