TBH Lucario isnt popular due to a lot of reasons
Hes really is not flashy at all. outside of some fair-nair-jab-fp mixups, his 'combos' are pretty standard. He cant spike people, his gimp game is as simple as spamming fair and has no KO moves which are cool to land (things like warios waft, sweetspot falco fsmash, snake uair etc). When Lucario matches can often go for around 5:00, you wont find people wanting the character who drags out matches this long to win. When youre a borderline high tier character only, these reasons dont give people much incentive to play him considering how much work you have to put in. Then theres the problem of people disliking playing against someone who uses lucario...
from the enemies point of view, playing against lucario isnt too dissimilar to someone like G&W. the entire game can be spent trying to get past his walling, only to end up falling into his auto-combos. having a full charge aura sphere severely changes the way people approach. every single second of the match has to be spent paying attention to Lucario or youre gonna get hit by it again and again and again. When he refuses to die due to his crazy good MC, DI and weight and manages a KO and takes off around 60% in 3 attacks next stock, that really infuriates people. and of course everyone hates getting hit by the dair. challenging it is rarely worth the risk and it just makes this huge zone the enemy will never feel comfortable being in because no matter how well they space udnerneath him, dair to stall can ruin a lot of attempted KO set-ups.
Almost every single person ive ever played absolutely hates playing against my Lucario. they never elaborate why, but imo its a combination of all those things.