Because it is very profitable saving method in last days. It grows a huge amounts in a hours and investors are having a good profit at everyday.
Making your bitcoin stay for couple of years inside your wallet is dangerous for you. It is only like saving cash in a bank and waiting the interest to get mature. Neither of these investments is good. It is really better to let your money work for you. Dont let your bitcoin reach maturity inside your wallet without even being circulated and earn double or triple that making it stay inside your wallet.
For some reasons, making bitcoins stayed inside the wallet for so long somehow doesn't provide satisfaction for me, it takes a long loooong time to take the target profit. Instead of just saving it we should consider also some things that might help to increased it without waiting for so long time.