I think it's because you've spent more time with older stuff that's had a chance to continue to grow on you. I always find I appreciate certain albums much more a few years out.
2019 wasn't the best year, but it also gave me Pony, Father of the Bride and Fear Inoculum. Oh, and AOKOHIO...and A Bath Full of Ecstasy...and Omoiyari...ok fine I guess 2019 had a decent amount of stuff for me after all. But if we're talking best-of-the-decade, Pony and Fear Inoculum are the only ones that I'd consider best of that artist from the decade (and in both cases that's their ONLY output from the decade)