When you lend coins on Poloniex you are lending them to real people who use them to margin trade. Poloniex charges the borrowers interest and gives you most of that interest. It's a real loan, serving a real purpose and earning real returns.

When you lend coins to Bitconnect they don't do anything with them, don't earn any returns, and pay you back with less coins than you loaned them since the price of their token goes up faster than the interest they are paying you. When the price of the token stops going up it is no longer profitable for them to offer loans so they cancel the program causing the price of their token to drop instantly. They end up with your BTC and the victims end up with worthless tokens.

Do you see the difference?