Investing only in BTC may not become profitable as investing in Alt coins. Btc won't rise 2x or 5x in short period as a lot of alt's easy can do. That's main reason investing in alts. But don't skip BTC too
On the flip side alts can lose you 50-90% of value quickly too, which Bitcoin does not. It is a bit of give and take in that regard, which is why I always say you should have Bitcoin and alts in a good profile.