James Wiseman

Wait shit this is an ancient thread, sorry, didn't notice

The mending and infinity thing is because they both allow infinite use, in different ways. Infinity allows infinite arrows, Mending allows infinite repairing. (to be clear, I also don't like this feature, but that's the reasoning).
Soul Speed and Depth Strider/Frost Walker don't have the related purposes like this, you can't use them to achieve the same goal. The basic logic I reckon is that DS/FW are overworld speed, and do nothing in the nether (because water), while soul speed is the opposite, only useful in soul sand valleys unless you specifically make soul sand paths in the overworld.