Having some understanding that we couldn't really just pick the right altcoin anytime we try, we are not that lucky, is the biggest reason. People are skeptical because there has been a ton of altcoins that caused a lot of trouble for people and that's the issue, we are not talking about something that has been clean so far and we have worries out of nowhere, we are talking about something that has been dangerous since the first try and that is why people are not entirely sure about it.
I mean if you have seen so many guns fired in a place, would you keep going back in there just in case the food is good? I doubt so, that's why there is a worry.
The vast majority of altcoins are unstable and highly-centralized, making investors think twice before pouring money on them. Bitcoin is a safer investment because it's the one cryptocurrency that started this craze. No other blockchain compares to Bitcoin in terms of decentralization and censorship-resistance. The fact that the supply is limited to 21 million units, is more than enough for investors to take it seriously as a long-term store of value.
I'd say altcoins are experimental grounds for testing out new features before they're rolled on the BTC blockchain. Developers will decide which features are worth it, for the community to review them. If you know how to play your cards right, altcoins are for you. Otherwise, I'd suggest you "stick" with Bitcoin for a while. No one knows what the future holds for alts, so we can only hope for the best. Just my opinion