Must be finding a guitarist willing and able to write to a song already written on bass. I think there are a number of factors at work. For one, it's not an easy task to do it well. Secondarily, most guitarists aren't interested in working off of someone else's stuff. Lastly, it just never seems to cross anyone's mind that songs can be written starting with the bass, even though the rock genre is literally filled with some of the most beloved songs that were written by the bassist.