I don't know why people are surprised. WTSMG sold over 5 million when it came out, it's probably on way more than that now. That's a big number for any UK act. They also made a lot of noise when they came out. And known worldwide as the frontrunners of 90s britpop, which was a massive export in the 90s.
There was always the same tired stories at the time about artists "not breaking in America" - it was such a trend in the music press. They made a big deal out of Robbie Williams flopping but he actually DID really try, and his label spent a lot of money trying to make it happen, so that story makes sense. But Oasis are not a pop act. They toured a lot - successfully, and had multiple multi-platinum albums. Rock audiences are just different - Nirvana never had several hit singles on the pop charts either but they're still considered one of the greatest acts of all time.
Oasis are an alternative act in America, so yes less mainstream, but I never doubted that they had a massive following.
They're also still massively influential on today's alternative scene. There's a reason they were nominated for this year's RNR Hall Of Fame.