They are building the line to the European loading gauge. The line is designed to be fully compatible with European very-high-speed lines, as DB and SNCF have expressed an interest in running through trains from European cities to Birmingham through the Channel Tunnel once the Eurotunnel monopoly expires. Additionally, the line is being built to serve very long trains (up to 12 European-length carriages).
All-in-all, the wider/taller loading gauge (which provides the option for double-decker carriages) and the long platforms will mean that HS2 will provide enormous numbers of seats between London and Birmingham.
It's not practical to upgrade most UK lines to the larger gauge, because it would mean rebuilding every station, raising every platform, re-laying every piece of track with wider separation (displacing hundreds of thousands of homes and businesses near train lines), widening every cutting and embankment, rebuilding every bridge, and reboring every tunnel. It would be possible, but not politically viable (imagine the voter response to being told that there will be no train service between London and Bristol for the next five years due to regauging, and oh by the way, we're taking half your back garden).