Because in my opinion, assisting Microsoft's acquisition of marketshare in any market is ultimately detrimental to everyone. They are a societal and economic evil, and directly aiding them is highly unethical.
Yeah, so how that's fundamentally different of any other company?
Do you really think that if Chrome would get 90% of the market share the web would be a better place?
It'll be just as closed. It would still have some remains of open source, but the technologies wouldn't work with anything non-Google.
It is my opinion that, based on their conduct past and present, Microsoft's existence and power is a net loss for the world. Google's past and present conduct lead me to the opposite conclusion. Naturally, I will treat these two entities differently.
Hypothetical future scenarios based on some rather extreme assumptions I do not agree with are not going to change that.
You are, of course, free to reach different conclusions, but the level of hostility and condescension I've received here for voicing my opinion in response to a direct question is disturbing to say the least.