Lucus seems a bit foolish which is probably why Leah has never taken him seriously (beyond the fact she's just not into him). One, she's been clear on her feelings. He may not be able to get over his just yet, that's his business, but he can't keep pushing her to accept his feelings either - its been years without sign of hope. Second, if he can't appeal to her himself, getting rid of the other guy isn't going to settle anything. Even if Kroemer gives up, that hardly equals her accepting him.
The other perspective is that he believes Leah is desperate to marry to resolve her family issue and thus thinks that if Kroemer backs out he'll be the best option left so she'll go with him. However, if that's the case that's rather scummy. If she preferred him she would have already said so, and he would essentially be capitalizing on her unfortunate circumstances and eliminating the competition to force a relationship with her. He may give off that innocent puppy image but he's not acting in her best interest but rather his own.