People invested money in them because it was possible to get profits because in 2017 everything that entered the market grew.
It was intense.... you could throw a dart without looking and the project it landed on would make you a few hundred percent profit at the very least, which is why money went into projects like never before.
When Bitcoin was done pumping and entering a corrective phase, altcoins in early 2018 reached their peak because a lot money went from Bitcoin to altcoins just because it was so easy to make money there.
I doubt we'll ever get to witness that again unless altcoins in their entirety pump, but that's not very likely with how people rekkt themselves hard.... they rather stay away and move on because they aren't done recovering yet.