ICANN (and ISOC and hundreds of other organizations) are structured as corporations under 401(c)(3) of the US tax code because that's the only way to set up a non-profit (apart from a Trust, which is much less transparent).
If ICANN had been set up in another jurisdiction, such as Switzerland or The Netherlands, which were both considered back in 1998, it would have been a non-profit Association. But at least in the Netherlands, the chances are that it would have been subject to VAT anyway. In the UK, it would have had to argue for Registered Charity status.
Since the US Government insisted in 1998 that ICANN be subject to US jurisdiction, there was no choice anyway, As Kieren says - it is what it is. The least bad solution, like democracy.