I have to honestly say the question puzzles me. Biologists ask 'why' questions all the time. All scientists answer questions as specifically as their access to the phenomena in question allows.
I suspect the question really means "Why have biologists been less able to unlock the functionality of their studied phenomena less specifically and thoroughly than physicists?" Biological systems are the most complex examples of physical organization known - IMO it's because biologists have the tougher job, in that regard. Exactly modeling every physical system of every living organism and the effect of every aspect of every gene on every cell in every organ (one DNA 'molecule' has more atoms than there are stars in our galaxy) simply can't be done.
What a lot of people don't seem to grasp is that this lack of thorough specificity on the most fundamental level does not imply total ignorance of biology. In many cases, biologists look for larger correlations between associated phenomena; they test their theories and make discoveries that work in repeated applications. The bottom line is that successful theories stand strong amidst an influx of new data; whether or not we know every minute detail of why the theory works is irrelevant as to whether or not it is true (though such details make for good future research). Biologists, just like physicists, are as specific in their conclusions as their data allows. (Pseudoscientific theories, in contrast, tend to lead research away from specific explanations toward inapplicable generalities.)
There is definitely an element from both within and without the educated 'elite' of the physical sciences (i.e. physics, engineering, etc.) that show disdain for biological research as an 'inexact' science, just because of the inherent limits on the systems they study. We even occasionally see mathematicians & engineers post on these threads with an implicit (or explicit) condescension towards the biological sciences; meanwhile, a great empire of advances in medicine, genetics and biochemistry continues to mount up and even affect our daily lives. Such people, though likely brilliant at what they do, approach science with the attitude of a race horse wearing peripheral blinders.
I think this attitude largely comes from Freepers (and perhaps others) who are neither scientist nor engineer. Time and again we see posts claiming that physical theories are proven while biological theories are not. And posts claiming that physics and chemistry (though curiously not usually astronomy) are somehow more rigorous than sciences like biology and geology. Such posters usually resort to whining or obfuscation when asked to back up their beliefs. I haven't encountered any real-life physicists or chemists who claim that their sciences are more well-founded than biology, paleontology, archeology, etc.