ArbitraryRenaissance

I know this is a 6 year old post, but I found the answer after a bit of digging. It's because gallium and other liquid metals oxidize rapidly. The material that wets the glass is not the base metal itself, but the oxidized form, which understandably will have relatively stronger intermolecular forces with materials like glass due to polarity. Mercury is more resistant to oxidization, so it is less prone to wetting.