Bitcoin will Hit 20000$ in 2018 may be because it is very famous currency of the current time and the main thing is the very special news coming soon, According to this Bitcoin will list on the top one exchange of USA, if this case approves than it will touch 20000$ and make a new record.
Even if bitcoin is very famous. It does'nt mean that it will grow again up to $20000. As you see the market all cryptos are struggling. I believe that bitcoin will reach its all time high again. But i think it will take time before it will reach $20k again.