Why does a bitcoin client from 2015 sync to the BTC chain if BSV is the unbroken chain?

Accepted: It is programmed to seek out the BTC chaintip

Each time the Bitcoin protocol has been migrated (to the Bitcoin Cash network and then to the BitcoinSV network), the block propagation protocol has been updated with a new ‘Network Magic’ identifier. This is what identifies all Bitcoin blocks as blocks on their given network. Your 2015 client will only see blocks that have the network magic identifier for the BTC network, whereas a BitcoinSV node will know that all blocks after the split have an updated magic ID at the front.

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.

Consensus Rules aren't Philosophical

Defining “bitcoin” is a philosophical exercise. BTC and BSV has specific consensus rules that the mining software checks, and if the rules don’t match, a block is rejected. However, if you sync up a BSV node, it does sync back to genesis in 2009. It just don’t sync with the Bitcoin Core network.

Another example is to think of it the other direction. You can’t send Segwit or bech32 coins to a BSV wallet because BTC breaks bitcoin’s consensus rules on the BSV network.

So why is BSV bitcoin? Because it rejects Segwit coins, P2SH, RBF and a whole host of other things that are not bitcoin! That does not change the fact that the chain with the most hash power and PoW is BTC because honest nodes are not securing it. BTC has been under a sybil attack for almost 3 years, and there are no honest miners willing to fix the problem of UASF.

Give this a read: https://etched.page/e3fd939b1ccb2d7ab3670091363b2e468a53b23df92c9f634c588f01784d5f0c/https://coingeek.com/is-bsv-the-real-bitcoin/

Miners with most pow chose to "upgrade" to the protocol change.

Basically the majority hash of miners with the most pow downloaded the new segshit software. The miners who chose not to listen to the segshit chain were the minority. Had the opposite occured, who knows what could have happened. Bsv is the chain with the unbroken protocol. That’s what really matters.