Cause altcoins are way too cheap than bitcoin. And it also give you more opportunity to gain big profit though you need to take some risk on investing on alts. By the way I can suggest you 1 alt that will give you good profit If you hold some of it. Try to check Privcy I believed that this coin has a big potential in terms of value and uses. Maybe this is not the 1st privacy coin exist in the crypto world but It is worth trying for.
Altcoin is far more cheaper than bitcoin and will have the chance of big gain rather than bitcoin which have small percentage of increase some altcoins can gain up to +50% which you can get profit easily come to think of you can buy atleast 1000 units of the coin and only a partial of bitcoin if you will compute the percentage of the profit you will realize that the altcoin is far more profitable than buying a partial of bitcoin and wait to sell it.