Comment by Analemma_

People have answered this before, and what it boils down to is: because they took VC money, and then went public. If you bootstrap a business from nothing and grow it on its own, then sustainable growth and stable profit is fine. But Twitter got VC and IPO money with a promise that it would have Facebook-like growth in return. I think the analogy I saw used was that they promised investors a Porsche and instead delivered a Toyota Corolla- it might be solid and reliable, and perfectly fit for purpose, but it's not what they promised when they took that money.