I've never understood why Sustrans, Cycling UK don't work together putting cycling as regular transport for all. The name change seems pointless and costly for little if any gain long-term.
It's a consequence of history. Sustrans's precursor Cyclebag (of Bristol and Bath Railway Path fame) was formed in the 1970s/80s when Cycling UK's precursor CTC was being especially stupid and short-sighted in opposing non-motorised routes due to a fifty-year fear that it would result in cycling be legally banned from more carriageways if there were more cycleways (a lack of confidence in their ability to make the argument that nobody would need to be banned from carriageways if the cycleways were any good, IMO), rather than what actually happened which is that cycling was bullied off most A-road carriageways and "designed off" new "improved" dual-carriageway A-roads like the "improved" dual-carriageway sections of A5, A43 and A46 in the Midlands, without even rubbish alternative provision, let alone anything good. Lots of other cycling campaigns were formed in other cities and towns around the same time: I think LCC, Camcycle, GMCC, Pedals and Spokes all started in that period.
So CTC was primarily about riding, while Sustrans was primarily about building. I think history has shown that both approaches are insufficient, but Sustrans's was probably needed more at the time. Until CTC converted to a charity, it was probably legally awkward for Sustrans and CTC to unite. Even now, their aims and structures may be different enough to make it difficult, so is it worth spending resources doing it? I don't know. They used to work together in the 2010s as part of the "Active Travel Alliance" but it didn't really work well. Not heard of it? I'm not surprised.