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.
Investing bitcoins can give you double or triple profits on your investment but you cannot ignore the risk factor attached with it and if you invest at wrong place and if your investment fails then you are likely to make big loss and comparatively holding is bit safe as people have made huge profits only by holding and even in future there are higher chances of making big profits only by holding as price will continue to increase depending on the higher demand of bitcoins.