I have moved my old answer to the end. It doesn't matter now as @bli's answer covers all my points and is better. Here I will give a retrospect of biostar, and explain why some of us are enthusiastic with this new proposal.
Biostar was originally a bioinformatics SE1 site created in ~2009. It attracted many good bioinformaticians who had contributed numerous high-quality answers. In 2011, the company behind SE offered to migrate biostar to SE2. The biostar community was very positive, but the maintainer wanted to implement his own Q&A backend and decided to leave SE. In the first couple of years after the transition to its own codebase, biostar was still ok, but later it gradually lags behind SE. Biostar was developed largely by one developer. As good as he is, he can't single-handedly compete with SE. As a result, biostar lacks many useful core features of SE. In addition, biostar aims to create a friendly community (which is highly appreciable), but it is too tolerant with poor questions by design. This, IMHO, alienated some top contributors who want to learn from high-quality answers given by others. Nowadays at biostar, several top users are answering most of the questions; questions outside the domain of sequencing are rarely answered.
Compared to many new SE communities, the bioinformatics community is relatively well defined and more experienced with Q&A sites. It is (at least was) also an active community, so active that SE2.0 were willing to adopt biostar without the area51 stage. I don't see overlap with SO and other SEs pose any issues. The past area51 proposals all failed solely due to the existence of biostar, not because we didn't know how to "define" a bioinformatics Q&A. Then what has been changed in this new proposal after a year? Two things: 1) more biostar users might be thinking to move; 2) the new proposal got more exposure early on. I am cautiously optimistic.
The following is the original answer:
When you ask a question on a format, a tool or an algorithm in bioinformatics, the only relevant SE site is biology. Asking the question elsewhere would receive few answers as people there don't have the background knowledge. You may ask at stackoverflow, but the site is flooded with too many programming questions. Few biologists care to join. As to biology SE, the bioinformatics tag is not very active. Questions, answers and votes are fairly infrequent. We don't have a sizable community there. If you could find a way to attract more bioinformaticians, biology SE would also work for me.