FACTS:
Blackcoin is fully mined within the first week.
The author of Darkcoin insta-mined 30%-50% of existing coins within the first few minutes.
Darkcoin's so called DGW difficulty adjustment algorithm is intentionally set to increase difficulty slowly after the initial launch. The first 1000 blocks are generated within one hour, and each of which has a block reward of 500. Remember currently Darkcoin block interval is 150s and block reward is only 5.

Why people say Quark is a premined scam coin, while they give Blackcoin and Darkcoin so many credits?

IMO, most coins launched after last December are scam coins. Their authors created these coins for cheating people and making money.

Quark is one of pioneer CPU coins. It was so cheap before and thus was manipulated by some Chinese whales last December. Right now most speculators have left the market and price is stable. There is nothing wrong with Quark itself.