Maybe it is because of the fact, that AS SOON AS you want to withdraw fiat you have to answer a whole bunch of questions (AML / KYC) like for example where you exactly got the coins from. If mined you even have to prove you have purchased mining equipment by sending some invoices. If you do not answer in a way they like you to answer, I think the coins are kept for "security reasons". Not sure about this point though, I just think I have read about that somewhere.
This is a new rule, which was recently introduced (and probably scared many people away).
I for example went to kraken, because I will BY NO MEANS justify what I do with my coins and what equipment I have bought.
Imagine I run a business selling used, worn panties. Why should I ever explain this to anyone on Bitstamp? It is my private thing, and this is basically the reason why I moved away to kraken.
The issue goes much further. Even IF you have the invoices for the mining equipment, supposed you were a successful trader and managed to increase your holdings tenfold on btc-e or other exchanges they will probably ask you for proof of that as well.
I doubt many have kept all the logs needed to prove such.
Likewise, asking for proof of having purchased some mining gear is nonsense. Anyone who purchased an AMD 5xxx/6xxx back then when BTC was below a dollar still, could justify any amount of BTC even if he did not mine at all.
On the other hand, some who purchased mining gear might have sold their BTC early and the BTC they are holding now is from evil laundered money.
So next they will ask you to show them which wallets you mined into a year+ ago... and if you did not log that data or lost the old wallets for whatever reason, you are screwed again.
They need to give you all this info BEFORE you sign up, not take your money and BTC and then trap you in. This is just shady business there.