If you go out and build an amazing strategy for, say, index futures, all you have to do is set up an account at the futures exchange. If you make a bunch of money the exchange doesn't care. The exchange isn't your counterparty, so your profits don't come at their expense. They're just happy to execute the volume.
Whereas with currency, most trades are done through over-the-counter "liquidity providers" (LP), traditionally major international banks. If you make too much money, the LPs get pissed off. Since they're acting as your direct counterparty, your profit comes at their expense. Consequently, a great trading system alone isn't worth that much in currency space. If you make too much money, you'll just get banned from all the LPs.
Like many others who tried to break into the space, XTX already had great HFT trading models. But their real innovation become an LP themselves. Rather than beg the major banking franchises to give them a seat at their table, they went out and built their own. That required creating business relationships with upstream sources of order flow, like retail forex brokers.
That customer base gave them the capability to capture market share without worrying about counterparty risk. Combined with their superior HFT models, it gives them a major edge over traditional banks and LPs. They can offer tighter spreads and still make a profit at a price point that their competitors can't.