SAIC's MG is a instantly recognisable brand, that with MG4 selling at a good price point and a very good car initself has made it very popular.Behind BYD, Changan is probably the most successful Chinese brand in terms of brand recognition. GAC is split between Trumpchi and Aion, while SAIC's MG and Roewe are basically dead in China. Same goes to Geely, GWM, Chery etc.
But the main reason is because Changan is a central govt company, while GAC and SAIC are owned by provincial governments. I'd assume any such consolidation move by Beijing would favour Changan.
Provincial governments will be pissed but they'll have to deal with it. They can try converting factories to parts suppliers, etc. Do whatever they have to do. It's not like GAC and SAIC are doing very well under the current situation anyway, since BYD is destroying the profitability of their joint ventures (Toyota, Honda, VW, GM) while their factories for their own brands are way underutilised.
Yeah, it's a very hard thing but I think it has to happen eventually. The central govt needs to put in more effort even if it pisses off regional actors. So many SOEs in other industries have already been merged to form competitive megacorps while the car industry is still a mess of small uncompetitive (globally) companies.