Why tamoxifen is preferred?

If fluvestrant is more potent than tamoxifen and more safer than tamoxifen, then why it is not drug of choice for ER+ breast ca in premenopausal women?
Why tamoxifen is preferred?

See the best batsman is not always the opener; first down is a very imp position.

If tamoxifen is ineffective i have fluvestrant as an option left.

Beautiful example Sir:)