LaGuardia needs to break the hold of the Commonwealth Party over much of the South, so that civil rights can be upheld there (unless you're planning to let people like LBJ to reform it from the inside).
National Labour's so smashed apart at this time he's pretty much the only ine interested in leading it.Finally read this, really enjoyed it.
Initially quibbled over Mosley ending up leading National Labour, but he was apparently initially on good terms with MacDonald, so I suppose butterflies could see that continuing. It still seems rather out of step with Mosley's OTL positions at the time though.