Because hashrate of AsicMiner is increasing and every share gets more hashing power over time.
With 100TH (now 200TH) bitcoin mine you get 400MH/s for every share and that is it.
Let me explain really simple.
You buy 1 share of 100TH mine for 0.2BTC.
In 1 month your 1 share have produced 0.05BTC.
But price of share has dropped to 0.1BTC.
Another months passes your earned additional 0.03 for total 0.08BTC, but
share price has dropped to 0.05BTC and is now worth 0.05BTC.
Total earned 0.08BTC with share price at 0.05.
Or 0.2BTC invested 0.13 returned.
Known since before 2012 as "
the problem with PMBs."
I still don't see why people are buying any form of PMB! or even when buying hosted hardware is cheaper.