Well the first thing that I thought of after the fireworks started was – why doesn’t he let Lewis past? Then I realized two things. First, Nico did not see how bad the situation was, second, he was still going very fast.
I think the key question is: Why didn’t Lewis pass on his own? I see two possible explanations.
a) Lewis was not fast enough. But this would also mean that Nico was not too slow either, so why moving aside?
b) Lewis could pass, but preferred not to for some reason. Could be safety, could be something else, but this very reason – if we knew it – would probably also explain why Nico need not move over.