Do you think the primary reason they are so slow to expand coin selection is extreme caution as to what coins to add or is it more technical restrictions and making sure they are able to service the new traffic of a new coin?

If they're concerned about only adding valid coins I think we could see a handful added relatively quickly including those in your post. If it's the latter technical concern I wonder if they'll stick with one at a time and add them very slowly like they have been so far.