Forget Chapter 11, Mero practically spells out the premise in Chapter 2 via Remilia. Reimu doesn't know how to deal with her feelings and is much less willing to seek out or listen to relationship advice from her friends than Marisa is (and Marisa is pretty reluctant in that regard herself!).
And like, look at page 39 of this chapter! Reimu is realizing that she was completely mistaken in Chapter 2 - she can't, in fact, just let go of Marisa. Reimu is not afraid of confrontation or speaking her mind; this is partly leaning on Reimu's established personality from the canon works, but I think it's established well enough in this manga too. It's precisely because she cares so deeply about Marisa that she can't give a clear rejection, clumsily tries to avoid acknowledging what's happening, and ultimately (and uncharacteristically!) breaks down in tears when Marisa refuses to play along.
