If an altcoin brings something new to the table, its adoption is easily-explained. For example, if a coin was released that combatted extreme gains by early adopters, the coin would server a niche, or a particular purpose.
Adoption of the "scam-coins" as some people like to call them (coins that just have a few variables changed) in my mind is simply market diversity. Just because Hershey's makes chocolate doesn't mean we don't like Nestle. Same with coins, All alternate currencies today are viable for just the simple reason that they split up a market further, which can be looked at as both good and bad. There's nothing wrong with these coins, and they are based on tried-and-true code.