Just because someone places hashtag on his twitter account doesn't make people go and buy bitcoin.
Elon isn't just a random guy. It's not for nothing that he has succeeded in his auto endeavours. He's an influential billionaire with a modern day touch. Isn't he the wealthiest man on earth at the moment? We should be in awe if his action shouldn't spark a kind of reaction on any issue, especially tech ones. A lot of investors trust his sense of judgment, seeing his electric cars and the wonders he has been able to perform there at a time when it is almost impossible to have such autos run on earth. Putting Bitcoin on his profile shows support and that means he too should stack in possession. If he has, that means (by conclusion) Bitcoin is an investment worthy to make. I guess all that factored in is the reason for the sudden bullishness in Bitcoin. That's quite understandably.
Elon is a powerful guy and an influential icon, even the youngsters (like me) follows him, not because of his knowledge and being smart alone, but because of him being an all-around person, and investors would definitely trust in him, although I have to admit that he can be a part of this spike, but not entirely him being the main reason.