That's one question I don't understand
I see there are thousands of alternative coins and most of them are forgotten or just not being used anymore.
Any logic reason?
I believe most are developed while in a pump-and-dump stat-o'-mind. But, that doesn't need to be the case. Soon, new alts will break out outta said mindset and will be able to preform under their own merits. Some will perform better than others based on their fan base or how heavily they're promoted (sometimes both). A lesser know entity backing an alt could easily out-perform a well-known entity backing their coin, again depending on the mindset of their respective dedicated fan base, coupled with marketing. Apologies for stating the same thing twice, but it was worth repeating.
I see alt-coins obscurely named after animals, colors, memes, etc.. pre-mined, mega coins mined per minute or less, unlimited or hundreds of billions market cap (mined, not value), based on some Z-17, et al. algorithms, going the way of the dinosaurs.
Don't get me wrong, with everything currently wrong with the alt coin space we're experiencing, a lot of leg work was already done to glean valuable information from so that future alts would have a better chance of surviving and bettering humanity on some level.