Why are so many small private jets sold just after a few years?
In the United States, aircraft that are owned and operated by businesses are often depreciable for income tax purposes under something called the Modified Accelerated Cost Recovery System (MACRS).


  This is beneficial since faster acceleration allows individuals and
  businesses to deduct greater amounts during the first few years of an
  asset's life, and relatively less later. (Emphasis mine)


But even more so, The 2017 Tax Cuts and Jobs Act provides for 100 percent bonus depreciation, allowing taxpayers immediate deduction of the cost of aircraft acquired and placed in service after Sept. 27, 2017 and before Jan. 1, 2027.

This means that perhaps if someone were to have a financial windfall (sale of a company, or a large sale of highly appreciated stock), they could purchase a rather expensive airplane, and if used for business purposes (generally at least 50%), they could offset their taxes for the entire purchase price of the aircraft in the first year! 

To have an overly simplistic example: Say you had $10 million in ordinary taxable income this year. You could reduce that by purchasing a, oh let's say a (2020 List price approx) 2.6 million Cirrus Vision SF50 Jet. You'd be able to reduce your taxable income by 2.6 million and pay taxes on "just" 7.4 million.  Now say at least one year later, you could sell your jet and pay depreciation recapture long-term capital gains tax (which depends on the amount of your ordinary income and is capped at 25% ... which is less than the 2020 37% top tax bracket).  This would have saved you in this example 312,000 (12% of 2.6 million) in a year. Of course, I'm sure it's a little more complicated than this oversimplified example, but you can see how it might be advantageous from a tax perspective, if you have a large income where you can benefit from the 100% depreciation in the first year.

So yes, it could be a tax play. Or it could be like so many things, what you thought was fast/big before you got it, now you have it, and you start to notice faster, bigger things and want to upgrade :)