If we ignore the angle of "we need to prolong the story so we can keep printing chapters", there are a number of characters with reasonable motivation for preserving the "status quo" of a non-involved relationship between Tomo and Jun.
Misuzu is the most obvious culprit, as she has her own difficulty expressing her feelings for Tomo. Tomo is, to put it simply, Misuzu's dearest friend, and Misuzu would like nothing more than to keep Tomo for herself. However, she cares for Tomo enough to want to help the romance with Jun, but she will still have misgivings throughout this endeavor.
Jun is another obvious culprit, as we have seen that he is at least subconsciously aware of his changing relationship with Tomo, and his semi-subconscious fear of losing the great friendship that he has been used to for years prevents him from exploring what could easily evolve into the perfect romance.
In the most recent development, even Tomo seems to share this fear of risking a good friendship for an uncertain romance (although all outside observers see that the two are a match made in heaven).
As any reader of romcom manga will tell you, the ultimate goal of the author and/or editors of this manga is for it to go on indefinitely without the audience getting bored of the romantic impasse. The preservation of romantic/sexual tension is the primary thing that will keep people reading, and if/when that tension is resolved, the wisest thing to do is end the series immediately because it will, without a doubt, seen to drag on if it attempts to continue after such a resolution.