My main reason for this was that the paths to success in academia are basically to be a glorified manager (aka. a professor) or a teacher. Doing research seems to be mostly not on the table for either path as that is outsourced to Ph. D. students and post docs on temporary contracts. The path to professorship is long and hard and involves dealing with organizational politics to establish slow progress via post doc positions, an eventual tenure track position to maybe a professorship. Over the course of this career you have progressively less time to do research and you get bogged down in bureaucracy, budgets, and other management stuff.
The alternate track of becoming a teacher seems reserved for those that fail the first path. This has always felt a bit wrong to me as an important function of a university is teaching. But the reality is that as a teacher, you do no research and you report to professors that are mostly interested in doing research and regard you as a second rate citizen. This seems to be more true in Europe than in the US where quality teaching is key to university revenue and thus your career.
I was well on track with the research path having a steady stream of publications and options for continuing. But neither path appealed to me and I found the prospect of dealing with university politics, funding, and publications for the next forty years to be unappealing. Teaching could have been appealing to me but not as outlined above where you are essentially treated like a career failure.
So I left university to work for a small software company (for the first time practicing what I had been preaching) and later rebooted my academic career for a few more years in industry by joining Nokia Research in Helsinki. In there I found myself doing a lot of the same things as a post doc but with a decent salary and career options. I had a lot of fun and worked with some smart people on some cool projects. Unfortunately Nokia did not last and I transferred out into the maps unit in Berlin. After a few years, just before it turned into here.com, I joined the startup scene and have also done some freelancing.
I haven't really looked back. I enjoy building software and learning new things while doing that. I'm currently in a CTO type role and I haven't published anything this decade. So, my academic career at this point is pretty much over. I don't miss it but it is definitely been educational and formative. IMHO the most important function of a academic education is learning to think for yourself and the ability to learn new things. That doesn't really stop when you leave academia and they don't have a monopoly on this. Especially for computer science related fields, a lot of the progress is actually made in industry, not academia.