He was outplaying him at the time. It's been a year and Randy Arozarena had a record-setting October, but if you look back at the stats that Lane Thomas brought to the majors from Class AAA or from the most recent season, it's not he was struggling. He was surging. Consider these four sets of production, sorted by OPS:
A - .270/.342/.472, .815 OPS, 16 HRs, 44 XB, 113 Ks, 396 ABs (28.5 k rate)
B -.264/.333/.489, .823 OPS, 27 HRs, 56 XB, 134 Ks, 515 ABs (26.0 k rate)
C - .285/.372/.454, .844 OPS, 17 HRs, 44 XB, 110 Ks, 533 ABs ( 20.6 k rate)
D - .344/.431/.571, 1.003 OPS, 15 HR, 44 XB, 71 Ks, 343 ABs (20.8 k rate)
You may be able to eyeball who is who is, and maybe even when is when. But A is Lane Thomas' career at Class AAA. B is Thomas' 2018 season. C is Arozarena's career at Class AAA Memphis, and then D is his 2019 season, overall. Arozarena exceeds Thomas' production. He does so with the added bounce of playing during the rabbit-ball season in Class AAA, and still his production stood out, but that's part of it. And Thomas had the edge defensively for the Cardinals, who knew he could play center field. Arozarena had the speed edge, though the baserunning scores may have depended on who you asked. Thomas is also younger, though not by much.
I bring this up, just to point out that there was evidence that Lane Thomas was a rising prospect, right there alongside Arozarena, if not ahead of him. The Cardinals even nurtured the notion at one point that Thomas could be their starting centerfield for a good stretch of 2020. He struggled this season, for sure, and some of that may have been related to COVID-19 and his recovery from the virus, the Cardinals have later learned. But let's not rewrite the backs of baseball cards that show he produced pretty well, too.