Its not that strange: people use what they are familiar with. Most people have a Discord account these days and migrating over there is as easy as clicking an invite link. In contrast Lemmy is relatively unknown and untested to the general audience, and is a step higher on the hassle scale, even if it is a similar service to Reddit - not counting the usual fediverse complications.
People are drawn to go as far down the hassle scale as possible, the fewer steps between them and their goal the better.
Not that a lot of communities did successfully migrate over here, partially or not. Lemmy is a lot more active now than when I started looking into it during the initial API struggle in June.
But then again, startups probably aren't considering the geo-political implication of country TLDs and that in 5-10 years any specific nation might simply stop existing.