Besides the network effect, there are another things that altcoin proponents ignore. Bitcoin has the strongest team of developers among all cryptocurrencies, as well as security vulnerabilities being fixed at first on Bitcoin (Litecoin/Peercoin/Dogecoin/etc will receive them much later). Recent OpenSSL issue is good example: Bitcoin is already fixed, Litecoin and others still vulnerable.
Forks can offer some nice features, but when it comes to the security (and a chance to lose all your coins due to unfixed bug), large investors will make appropriate conclusions IMHO.
It's interesting that NXT is not affected by this recent OpenSSL issue, because it runs on Java. At least that's what developers claim. Oh well, it's not a fork after all, but I just thought I should mention.