The biggest reason is access to capital. There's lots of institutional investors who are going to primarily invest in US equities, so if you're on the US stock

The biggest reason is access to capital. There's lots of institutional investors who are going to primarily invest in US equities, so if you're on the US stock market that's good. Even if those institutional investors do look at worldwide equities they're going to be limited in how much they allocate to it. It's also easier for investors - a single regulatory environment, no currency risk etc. This used to be mitigated by the fact that other countries would have their own pools of capital, like domestic pension funds but with the reforms to pensions UK pension funds are no longer a particularly good source of capital on the UK stock exchange.