Because of three things that have come together in time.:
Ability to run gene sequencing fast and cheap
Ability to GMO and grow out cells
Improved knowledge of immune system gained, in part, from the study of AIDs
We are in a biotech boom and a lot of wealth will be created over the next decades. Yes, there will be big corrections along the way. But we have not seen anything yet. 85% of the medical spending is on 15% of the world's population. These targeted therapies are going to be huge money as the rest of the world can afford them. We are going to figure out how to do a lot of things. We can finally begin to see how things work through the technologies above. These technologies will continue to improve and speed up. I even think that in the future the time it takes to run a drug trial may get cut in half or more.
Biotech will be just as big as the Internet Era, my guess is we are about 1996 equivalent in the Internet Era (in pricing). Where all the wise guys short things and get blown out.
There was decades of work and numerous technologies that came together in time for Robert Noyce and Andy Grove to be able to start Intel. We will see something like that come out of this era.
My use of the word bubble was a bit strong, but there is clearly no rational genie determining valuations. I have money in Athersys. I don't spend time worrying about why it has 1/20th the market cap of Mesoblast.