The nature of Hard Forks
It is because a hard fork is the only way to increase the block size once it is limited.
The 2015 node is programmed to reject blocks bigger than 1 MB. That is why it will never sync to a big block chain.
Other changes were added too, which also make it hard to sync to the new chains (BCH, BTC), but these changes would likely have never been added if a block increase could have been done with a Soft Fork.
That is why it doesn’t sync.