Because Tsukiko being unemployed and emotionally isolated has done a number on her perception.
Not only that, she also seems to have an inferiority complex towards Mei's mother (maybe her other sisters as well). I feel like we're due for some kind of flashback about how she ended up out of work based on that one panel in chapter 1 when she talked about not being able to work with others well.
I do think it's not
entirely on Tsuki-chan though. Sora probably has her own fair share of issues as well. For one, she wants to be very close to Tsuki-chan and support her but at the same time puts up a wall to keep her from getting
too close. She was super avoidant of telling Tsuki-chan what she did for work until this chapter and has noticeably never talked to Tsuki-chan about personal matters, her family, or even invited her to her home. This chapter sort of gave me the impression she was embarrassed about not holding a steady job and didn't want to ruin any sort of image she had in Tsuki-chan's eyes (also how did she keep that rabbit costume head? I don't think any company would just let her have it after unless she just walked out with it or something like that). Thankfully Mei gave her that jolt that Tsuki-chan's baseline for "friend" didn't seem too high and that getting closer to the girl you've been yearning for for 15 years is a two-way street.
Funnily enough I've noticed that Sora's more serious or somber faces are a lot more subdued than what we see from Saki or Kanon in Moon on a Rainy Night with usually only something like a sharper look in her eyes or her eyes being hidden which is pretty funny when this is the series that's supposed to be the gag comedy lol.