I'm generally a big fan of cycle spaces on trains, for reasons that need not be discussed in this thread.
But on the St Ives branch? I can't imagine there's especially high demand for bikes. How far is the ride from St Erth, three or four miles? A bit hilly perhaps, but so is any ride of any real length you could do from St Ives.
Besides which, the vast majority of people bringing bikes will be changing to/from other services which themselves have limited cycle capacity, so what would be the point in providing dozens of cycle spaces?
In this particular location, ceteris paribus, it's really not something worth considering.