Post Office Counters Ltd can't negotiate a contract with National Savings that's mutually beneficent to both parties. (i.e. where the Post Office makes enough money from the sale to make it worth their while selling Premium Bonds but where National Savings don't have to spend much more than they do by selling online). It's just the same as with TV licensing, where licences are no longer available through post offices because the economics don't work out right for both parties.
It makes perfect sense to me.