Many organisms do not experience the sensation of pain, because they do not experience anything. Pain, however, also means negative reinforcement. Pain in this sense is simply a reaction to any damage to the organism, motivating it to move or defend itself. But there is no experience. So far hedonism works alright, but now we move on to pleasure.
Pleasure inherently implies experience.
It does not have a second non-sentient meaning behind it as pain does. So since THE meaning of life must apply to all organisms, hedonism cannot work. Now one thing that ALL organisms do is live. You could argue and say that all organisms also avoid pain, to which I agree. Living is the fundamental drive, and pain avoidance is a subdrive of living.