Arsenal still have a really hot game to play come Wednesday against Real Madrid and that would definitely hinder them from actually winning that game against Brentford, because they would like to actually rest some players seeing that their chances of getting into the champions league semi is a lot higher now as they have beaten the champions already in England and still they know even that can be written off with an increase goal scoring performance from Real Madrid at home so it's better to stay cautious.
Arsenal has nothing to fight in Premier League with no chance of catching and beating Liverpool in the title race. After surprisingly won over Real Madrid in Emirates stadium with 3 goal difference, Arsenal know that they have very big advantage and they can join semi finals if playing a second leg very carefully and avoid a defeat in Bernabeu stadium.
Loss like 1 or 2 goal difference is still enough for Arsenal to go to semi finals, but by having no purpose for playing in Premier League, and importance of the second leg quarter final, they will skip a match with Brentford and reserve their best players for a match in Bernabeu.