There are many reasons, we've identified 5
We've written about the need to differentiate between goals, strategy and tactics. But there's more. You because you have a strategy and more or less follow it, does not assure you of success. Strategies can and do fail. There are many reasons. Over the years, we've seen five strong reasons why strategies fail, independent of the obvious reason of failed execution, which is different. A word... many strategies do not fail for a single reason, but rather, for a combination mixed with human error
Inertia - Doing what everyone else is doing. We see this a lot in both startups (everyone is using TikTok, we need to use TikTok) to more mature businesses (all my competitors advertise in The Boring City Mag! We need to be there.
Inertia - Dong what you've always done. We see this a lot in big or old businesses. Their marketing or advertising department believes that not much changes, so they execute the same strategy and plan year after year. Doesn't really matter whether or not market conditions have changed.
Lack of Data or Wrong Data - This is a real killer. Using the wrong data will lead you to the wrong conclusions. Having no data is the equivalent of guessing or rolling the dice. When not having data many businesses resort to doing what they did the previous year, though, of course, they had no data either.
Huge Ego which leads to unrealistic expectations. For example, assurances that the conversion ratio "will be huge because people love this product" or that your product is "untouchable"
Disconnection from Customers - This happens a lot, where companies do not believe in market research, or shun collecting and analyzing data. As a result, they might be completely disconnected from customer's changing tastes, or trends in the market.