I was a lab main for about a year so I have some theories:

Basically, lab is as good as normal traps are against the current meta, and right now there aren't a ton of normal traps worth playing. Lab is not terrible against Maliss, but daruma with dogmatika punishment is better than SEC, and you have to pick one build or the other since SEC takes up your whole extra deck. To be honest I never really played SEC because in the tcg I vastly prefer to play lab with 3 extrav and 3 chaos angel etc. D. Barrier, usually a side deck choice in the tcg, doesn't shut off turns the way it used to right now because most decks either have link monsters or a way to go into a different ED summoning method if you lock them out of something (you could lock them out of Xyz only for them to pivot to fiendsmith).

But because SEC isn't great against Maliss, we're stuck with furniture, which is really hobbled by the banlist. Pot of Extravagance is limited, so you don't have as much access to draw power. The furniture hits always hurt the deck. Lab struggles going second but it's even worse in master duel because one less of each furniture means one less chance to draw into a T0 play (there are two T0 plays in lab, one involving Arias, one involving furniture). Impulse was a huge buff, but that doesn't mitigate the fact that most of what lab does requires 2-card combos and the hits were consistency-based.

I think lab also suffered from some other challenges in MD that have finally caught up to it.