isn't that equivalent to asking why anyone would be foolish enough to not have 100% of their money in bitcoins given its meteoric rise over the past few weeks?

There are reasons to not have all your money in bitcoins, therefore there are reasons to sell bitcoins (even if you just bought them specifically for that transaction (for the anonymity for example))

edit: I realised this sounds a bit snarky, but I'm honestly curious. Is buying with bitcoins really that different from not investing in bitcoins?