The easy answer on why digital strategies fail is that technology is changing so quickly that once an organization has decided to implement one strategy, it’s time to alter course and develop a new one. And to a certain extent that’s partly true. It is very difficult to stay ahead of the curve, especially now.
But the real reason a lot of digital strategies fail isn’t because of a dearth of good ideas or talent, it’s because of 5 things inherent in a lot of organizations. They are in descending order of importance.
5. Turnover coupled with leadership vacuum.
4. Red Tape/Politics/Hidden agendas
3. Lack of belief or Buy in from Leadership
2. Poor planning and Bad execution
1. Culture
I suppose that we could easily expand this discussion to include other aspects of a business that fail instead of just digital strategies because of the above mentioned bullet points, but these 5 seem to occur moreso than most around aspects of digital.
For example, if we’re to believe the Gartner Hype Cycle, which I do, we’re approximately 2-5 years away from mainstream adoption of social analytics. That’s measuring and monitoring and analyzing the biggest technological and cultuural phenomenon to hit the globe since the internet first came on the scene, and we’re 2-5 years from mainstream adoption. It’s not a technology problem.
It’s tough to stay ahead of the curve when some struggle to just get on the curve. Why is that? Your number one answer? Culture. We have to get out of our own way.
Direct Marketing Observations 
Good post Marc. The scary answer in there is #2: If the planning (i.e. strategy) is bad, that’s a completely different issue. Or do you mean something else with planning?
I think one of the biggest reasons digital strategies fail is that many companies don’t really want strategies, they want some tactical executions. Most of the time, they are not the same things.
@Rich, great point rich about tactical executions, sort of like not understanding how hard work and planning go into becoming a championship calibre athlete for example, they just don’t show up and win..most people just see the prize or see the companies who have attained the prize and they want that, w/o understanding the sacrifice, the concessions, the teamwork, and the labor that it involves.