An issue with allowing to sort inventory by default is that it'd significantly slow down the already slow inventory fetching process. It literally needs to count how many instances of each item exists out of what you have - and also ignore those placed in a bunch of system accounts.
Not making it sortable by default but putting it as a separate feature would work, but that's more work that I'm not really willing to put into current website inventory system.

Not saying it'd never get implemented as the idea is good on its own, but very unlikely to happen until website inventory is reworked.