Business app projects fail for various reasons. One of the biggest is when the s/w fails to solve the right problem. That is, it doesn't do what the business needs. An article in CIO Magazine says:
Analysts report that as many as 71 percent of software projects that fail do so because of poor requirements management, making it the single biggest reason for project failure - bigger than bad technology, missed deadlines or change management fiascoes.
Requirements analysis is often a complex task, sometimes more difficult than building the app. It can be politically difficult as well, with powerful interests struggling over what an app should do.
Another problem is that often business people don't understand, or can't describe, what they do. Let alone how s/w could help them do that better. That's not because of lack of expertise. It's just hard to do sometimes.
This isn't a systems analysis course. All we need to know is that requirements analysis is often really hard. When you write a program to meet some specs, don't assume that the specs are correct.