Because the Brita is only designed to filter tap water. The Watergizzi does the following.
Removes >99.99% Virus—Meets or exceeds ANSI/NSF 53 Removes >99.9999% Bacteria—Meets or exceeds ANSI/NSF 53 Removes >99.9% Cysts including Cryptosporidium and Giardia—Meets or exceeds ANSI/NSF 53
I see the perfect application here on the east coast with the bike trails that go from Pittsburgh to DC. With this filtered bottle you can drink from virtually any stream, river, lake or rainwater.