There are some fundamental differences between Tether and Bitcoin that is the reason why Tether is stable to $1 USD, while Bitcoin has a fluctuating price. 

Basically Tether is backed by a reserve of USD held by the company Tether Limited. This is to ensure that the value of one Tether is very close to $1. In other words, Tether/USDT is pegged to $1 each. 

The SBD kind of had the same mechanism, but this is not backed by any real money, but rather by debt to the STEEM. 

The value of a Bitcoin on the other hand is entirely up to what people are willing to pay for it. There are not pegs or other mechanism in place to control it, and it's instead up to the free market. These days people are willing to pay a lot less for a Bitcoin compared to at the beginning of the year, which is why the price is falling. 

Since Bitcoin in controlled by a free market, it will never be able to be stable. Ever. The value might stay roughly the same for a while, but it will always change to a certain degree, depending on what people are willing to pay for it.