first is the previous two halvings that bitcoin had in the past 10 years. each of them had a big rise before the halving and after a month or two led to a huge rise.
and second is the fact that it slows down the production of bitcoin by 50% and that injection of new supply in the market changes the balance between supply and demand and can lead to a rise.
bitcoin has usages hence "demand" while altcoins do not. so when their supply is halved there is no demand to help any rise. instead what halving of an altcoin does is that it reduces the earning potential of miners and as they leave the coin loses security and value over time.
there are also a lot of other things in play when it comes to altcoins such as their pumped bubbles bursting, their hype being over, ... and in case of LTC it is a copy of bitcoin so it really isn't something special to have that much potential!