As already answered: Network effect.

The question is akin to asking why you can't modify the bitcoind instance running on your server to process transactions without debiting BTC, whilst you CAN do it, you will need to convince the super majority of users to agree that that's a good idea.

XCP acts as a anti-spam token, and an economic reward, if you tried to create your fork you would really just end up switching it to an XCP alternative token to fulflll those functions. Your newly created tokens properties (distribution, adoption, upside potential of continuing to hold or hoard) would have to match or usurp the previous

"We're aiming for a generic platform that is agnostic to underlying distributed technology, so not only Bitcoin but Ripple, Ethereum, Hyperledger, and a proprietary stack built around Counterparty are all in play," he says. "We're also working on private blockchains and 'known' networks, as we think that might be a better application of the technology for some institutions. The idea with smart securities is that the battery comes included: to create contracts that can act on the blockchain autonomously, from issuance and corporate actions to decentralized secondary trading, clearing, settlement and transfer."
Smith says the early target is illiquid securities and funding activities — including both traditional venture-capital and crowdfunded private-equity transactions, as well as other applications like merchant banking, intercompany funding, and syndicated loans. The firm is already integrated into multiple tier-one banks' development programs and blockchain pilots. Many are focused on smaller use cases that don't already have processing intermediation in place, relying instead on spreadsheets and fax, and at least a few should be implemented by year's end.
Symbiont aren't alone, with many others in the space crossing over to blockchain work now, but Smith argues there are "maybe three or fewer firms" in the industry that have both the financial understanding and technical expertise to do it right.
"Most people entering now are trying to execute on things like how to 'tokenize' a security, on that plane of the learning curve, which we were doing two years ago," he says. "Though that's an interesting process, to us, the real value proposition in convincing an institution to rip out systems, and change protocols, must go well beyond that."