because all these altcoins that are being created non-stop are not good projects. they are mostly shitty projects (hence the name "shitcoin") with nothing to offer and the rest of them also struggle with many issues which prevents them from gaining any real adoption and staying 100% speculative tool for traders to play around with.
Actually the profits from various ICO projects have been reduced and also most of the ICO projects turn out to be scam and just rum away taking all the invested money by various investors which gives bitcoins more importance.
Any newly coin faces several issues like listing on several exchanges, price dump, etc but as compared to bitcoins, bitcoins do not have any such problem and also bitcoins can be trusted much more easily. But most of the other successful alts can surely take the place of bitcoins soon if the network of bitcoins keeps remaining congested.