Many US-registered ETFs are not available in the UK/EU as they’re not compliant with UCITS regulations. You found the compliant version as it has UCITS in the title. More information can be found here
This speak to the smaller and less developed ETF market in the UK. On the whole we don’t self invest in the same way as they do in the US (at the moment, FT are gliding what they can to change this)