Because they're a fragmented complete mess of a company. Mainly because the unions are so entrenched that if anyone suggests anything involving efficiency then they'll all be out waving placards, so nothing ever gets done. There's probably someone whose job description says they do small parcels, another department that handles slightly larger parcels and another that deals with brown parcels.
Every business that unions get involved in always ends up bankrupt - history is full of clear examples, not least our entire car industry. They get high pay and good conditions for a few years, then they and all future generations end up unemployed.
The current billionaire owner isn't an idiot, he bought it knowing it's a complete basket-case of a company. But they own lots of land in almost every town and city centre, under all their sorting offices. They just need a few years to allow (and probably ensure) the company goes bust, then they can sack everyone, knock everything down, asset-strip and cash in.