Possibly because the dividend yield isn't as high as with other mining operations?
No doubt the low dividend has an effect on the price of the security, but investors need to realize that BTCMC is the only security that reports their finances. At least you know what your shares are worth if this operation closes. The other mining operations on GLBSE won't tell you this, but they are probably worth close to 0, and you will lose all your money if they shut down. This should provide security and increase the value of the company.