Because raising the blocksize limit is the obvious move that everyone knows should have been done long ago.  It's the simplest and most robust way to scale Bitcoin in the short term.  It avoids the needlessly complexity and risk of segwit, which was promised as a way to 'avoid a hard fork'.  Furthermore, BU removes the centralized point of debate from the consensus layer, which is how we got into this mess in the first place...so it lets the market decide this and decentralizes development.