Because those forks produce doubts about the bitcoin scarcity. If their name were not related to bitcoin it would be fine, but now some people will think that there are 2 kinds of bitcoin (21 M * 2), and that there will be 4 by the end of the year (21M *4). There can be only 1 true bitcoin but those alternatives produce doubts.
Which happens to be the max supply of LTC, 21m * 4 = 84m. So far 53m are in circulation.
This supply dillution might create equal prices for 1 LTC == 1 BTC talking fifty bucks.
When that happens we are dead.