On the horns of a dilemma: origin of idioms

Black/brown ram
On the horns of a dilemma

Life right now might be bland. Not much to do. And no people to see. Life during lockdown is colourless and lacks flavour. You might say it’s vanilla.

‘Vanilla” is Merriam Webster’s word of the day. Why does ‘vanilla’ get the blame for dull, lack-lustre boring and bleak, I have to add?