Why did some scholars rule that dyeing eyebrows is forbidden if it is an alternative to plucking, even though the basic principle is that what Allah and His Messenger (peace be upon him) have not forbidden is not unlawful?

The default rule for things is permissibility until there is evidence to prohibit them. Whatever the Shariah has remained silent about is pardoned. This is the doctrine of the Sunnis. However, the indication of Shariah texts for prohibition is not limited to the literal meaning of the words only; rather, it includes generality, the concept of agreement and opposition, and analogy. From this stems the disagreement on some contemporary issues, such as eyebrow bleaching. Those who deem eyebrow bleaching forbidden cite the hadith prohibiting al-nams (plucking eyebrows), and they believe that bleaching achieves the same meaning as the prohibited nams, which is altering Allah's creation.