Various reasons:
Sold my first audiophile speakers (Linn Kan MKI) and "upgraded" to Spendor S8e. Both outstanding speakers!!!!
After many years with the S8e, I "upgraded" to Audes Blues, which, IMHO, have been the best speakers I have ever owned.
Unfortunately I ended up moving my system to a much smaller room and I had to sell the Blues for a pair of Totem Acoustic Model 1.
About 5 years later I moved to a new home with a much bigger listening room and I ended up selling the Totem Acoustic Model 1 for, believe it or not, a pair of Swans Diva 6.1 which I lived happily with for a full decade. These speakers have been favorably compared to the B&W 804 Diamond (for a fraction of the cost). Trully remarcable speakers for the price.
Then my mother and sister in law moved in so my system is now on a second room; too small for the Diva 6.1 so I sold them and now I am a proud owner of the GE Triton 7s which, in my opinion, are unassuming giant killers if you value soundstage and imaging (like the Totem Model 1) but with surprisingly punchy, clean, and tout bass response.
However, I am saving, not in any rush but certainly saving for the best speakers I have ever auditioned, a pair of Spiral Groove Canalis Anima.
This time I'm keeping both the Triton 7s and the Canalis Anima.
To answer your thread, we all do it for the same reasons; trade bug, moving, budget, needing something smaller, needing something bigger, experimenting, changing ancillary equipment, you name it!!!!!
Happy listening.
