That's because it's NOT a day to be happy! It's YHWH’s most holy and solemn day of the entire year … a day of TRUE REPENTANCE about yourself, your life, and even the surroundings in which you live.
We’re supposed to be SAD that we might have HURT Him by our words, deeds, our living conditions, and even what we might have eaten that He considered “unclean.”
We’re not supposed to be “happy” while being immersed in the sin that’s all around us - even sin that's, perhaps, lurking in our own home….
We’re supposed to be be SERIOUSLY appealing to THE CREATOR who is searching our hearts, minds and souls to see if we’re TRULY HIS … or if we plan to keep making excuses about ourselves, and thus, pretending to be holy. (It's been my experience that there are MANY pretenders!!!)
It’s a day for total introspection and asking HIM what we should do next in order to CHANGE any conditions that make HIM unhappy with us in ANY way.
NONE of us can say we are perfect beings who please YHWH all the time. That would be a LIE! But this is why we have Yom Kippur - to repent and gain insight into how WE could change to better serve YHWH as the Holy Beings HE wants us all to be....