Why didn't the A330neo and A320neo share the cockpit of the A350?
The short answer is that it's what the market wanted.  There is  always a lot of discussions between the airlines and manufacturer's before Airbus or Boeing commits to building a new aircraft.  In this case both Boeing and Airbus came to the same decision -- the airlines were being driven by one single factor to replace their A320 or B737 fleets, they just wanted to reduce fuel expenses.  That's why the Airbus is the 'neo' new engine option.  

The airlines made it clear that any new aircraft purchases would be based on operational savings and that's primarily driven by fuel burn and the resulting costs.  The solution to that is a new engine.  Putting in a new flight deck is a huge expense for the manufacturers which would drive costs up and the airlines won't pay extra for something that doesn't save them money.  

There's no cost benefit to a new flight deck, especially when (as others noted) you have a huge investment in an existing fleet.  In fact, all it's likely to do is add costs.  With a new flight deck you'll have a huge increase in maintenance and spares costs due to the need for stocking more parts with new part numbers (just adding a part number to a logistics system costs significant money.)  You'll also need new crew training simulators due to the different displays and controls, whereas the neo option can be handled with software upgrades.  And if it drives a need for new type ratings, that would mean more training costs and making sure you have the crews with the right rating at the right time. 

In my experience in 20 years dealing with this market, airlines only spend money for two reasons: 


It will increase their profit margins 
The FAA (or other regulatory agency) mandates that they have it.  


Aside from that, you should consider Airbus' side.  They're trying to sell airplanes and they have to compete with Boeing.  Since the avionics architecture is quite different between the A320/A330 and the A350, moving to it would add huge costs and a couple years delay in entry into service.  They also have to deal with the fact that airlines don't want to see a big price increase.  Prices for the A320neo and B737MAX are not significantly higher than the current/older versions.  If the competition can offer a 7 to 9% reduction in fuel burn for approximately current prices, spending an extra 2 or 3 years to match that with a new flight deck means a lot of lost sales in the interim.  And you probably won't make enough margin to recoup the new flight deck costs.