If they're not high quality, said famous people are likely to sue or otherwise cause hassle for Wikipedia. That's something a rare flower is not likely to do.
The closest it's ever come was this incident:
http://en.wikipedia.org/wiki/Wikipedia_biography_controversy
where some guy's article stated for years that he was a suspect in the Kennedy assassination. He never tried to sue wikipedia, though he did complain a lot.
Incidentally, though, this shows another reason why wikipedia articles on borderline-notable people are bad: the fewer people look at an article the more likely it is that crazy non-facts will persist in the article.
[1] http://arstechnica.com/tech-policy/news/2008/05/dumb-idea-su...
[2] http://www.citmedialaw.org/threats/american-academy-anti-agi...
The standards for articles on bloggers and singers have to be high, because there's a huge number of bloggers and singers out there trying to promote themselves by writing on wikipedia about how awesome they are. Since nobody has the time or the inclination to police the article on Joe Blogg's Blog when it claims that "this is the awesomest blog on Earth", wikipedia tries to limit the number of articles on subjects where the article's editors are likely to have a strongly vested interest in the subject of the article.
Another reason: every obscure animal and flower species on Earth has been around for millions of years, and most of them will (I hope) be around in another million years. Towns usually have lives measured in centuries. The average blog will be forgotten (even by its author) in a few years.