Dont forget that BTC are limited, in a way well known.
By selling the pizza for 10000 BTC, he got somethign which he knew would grow harder and harder to obtain.

And, besides, probably the main reason is, he liked the idea.
You might think a free and unregulated currency is a good idea, and support it by risking the amount of 14$.
Or its just playing with science, like "whoo, how does this work, lets try it, it costs less than an evening in the cinema!!