Uxbridge is hemmed in by the greenbelt and has limited growth potential.
The town itself has a population of around 12,000. The town is not connected to the York-Durham trunk sanitary system (like Stouffville is), and relies on a local system. This system had maxed out capacity for many years, but recent upgrades have allowed development in the town to resume with increased capacity - albeit a relatively small amount of capacity.
Ultimately, the corridor to Uxbridge is winding and slow. A GO extension would be slow, would serve a limited population, and would not see significant use growth potential.
Once GO RER happens overall travel times on the Stouffville line should be a lot faster which may make it more feasible, but it would have to be an electrified extension serving a relatively small population.
Brantford is a CMA of 175,000 comparatively - albeit one further from Toronto than Uxbridge with weaker commuting patterns.
Collingwood GO would likely operate similar to Niagara GO service, with a heavy tourism tilt. There are around 60,000 people in Collingwood and Wasaga Beach, and it's a huge tourist destination with poor highway connections and higher regional traffic levels than a typical town of that size would have. The alignment for both corridors is also generally straight and supportive of competitive travel times if implemented correctly.