A redesign usually begins because the site or product feels dated. It ends with a new visual language, a new set of components, and conversion sitting roughly where it started β occasionally lower, because a familiar interface was replaced with an unfamiliar one and nobody measured which parts were working.
The reason is that appearance was the brief. Nobody established which step people were failing at, what they were trying to accomplish, or which elements of the old interface were carrying the result. Design decisions were then made on taste, which is unarguable and therefore untestable.
The other common failure is the reverse: beautiful design files that cannot be built as drawn, or that describe forty variations of a button with no system behind them. Design that ignores implementation becomes a negotiation during development, and the negotiation is usually won by whatever is quickest.