Because many members of the general public will presume one currency will win out in the end, rendering others useless. Exactly the same reason that so many people invest in so many useless coins here, because they are speculating that one "might be the big thing" and dominate the rest. For example people might invest in myspace without competitors, or they might invest in facebook without competitors, but many people may just hold out if facebook and myspace were competing against each other thinking on or the other would win out eventually. No one wants to back a myspace.
At one point in time MySpace was THE thing to back but time progresses, society evolves. Even if Bitcoin becomes mainstream and becomes the worldwide currency it won't be the last currency we ever use. Time progresses, society/technology evolves, someday there will be something better. Stop fighting it.
Bitcoin was made open-source to encourage alt-coins for people to speculate and experiment with. Even alt-coins with minor changes can reveal new things to better crypto-currency overall.
If someone designs a crypt just to test a faster block tme and we learn it works or doesn't work, that was of value.
If someone changes just the block reward schema and we learn that it makes the coin more or less volatile or influences the market to make it more or less heavily traded (like how DVC is worth nothing simply because it's block reward is so high) then that was knowledge of value.
Stop hating and worrying about whether your coin is going to last as others come out because one day, it won't.
I have no problem with alt currencies and genuine improvements over BTC. If people want to experiment with new things... awesome. What I'm saying is, if you're making an alt coin, make it an
alternate. Something like Bytecoin is a perfect example of what we
don't need. At the moment there is next to no real variation or innovation within the alt-coin sector. Generally speaking minor changes have taught us zero things that we didn't already know about cryptocurrency. Please list 5 improvements alt-coins have brought to cryptocurrency so far.
Again, no problem if BTC is eventually superseeded as it will be with something better, but you have to separate people complaining about pointless clones with no innovation which are just pump and dumps against some truly new innovation.