Due to their volatility. In my point of view, BTC will grow to about 10 000 USD per btc in next years. However, it would grow slowly, no more booms. Ethereum is more promising thank BTC since it is more suitable for regular use by general public. I don't see anything special about LTC, i guess it will be beat by ETH soon.
On the other hand there is Steem. If you are looking for investment, pick it. Its price will grow a lot soon.