A few years ago we had a close call with Bitcoin when Ghash.io mining pool became so popular that they nearly pulled off a
51% attack, but fortunately for us the Bitcoin community saw the threat and some miners moved their hashing power to
other mining pools. You can read more about it here :
https://en.wikipedia.org/wiki/Ghash.io The problem with smaller
Alt coins is that they have less hashing power, so a bad actor with a lot of hashing power can attempt to do a 51% attack, if
he shifted that hashing power to mine that Alt coin. Most people/Pools do not do that, because it will destroy the value of the
coin, if it is detected.