Oh this is easy. Because only altcoins can give you as much as 1000% profit in just a few days. Bitcoin trading is good but the profit is lower at a time and cannot be compared to that of altcoins. If I'm going to get a lambo it's going to be through altcoins investment and not bitcoins.
Also, tell people about the other side. You are right on the part that these alt coins can be more rewarding in comparison to bitcoins but they also come with more risk. Instead of getting your Lambo, you might have to sell your current vehicle to recover the losses made on these alt coin investments.
I am not saying that bitcoins are damage proof as prices decline for them as well but the probability is more in case of alt coins. Extent of damage will also be more in case of alt coins in my opinion. You can see it from the charts for last 4-5 months.