It's both cost and convenience. It can add up to £50 or £60 for hold baggage depending who you fly with. Add on extra waiting time in baggage reclaim and suddenly a quick cheap business trip becomes an expensive one with extra hassle.
Airlines need to encourage and incentivise hold bag loading by reducing the charges and reducing the waiting time at the other end, if they want to avoid the cabin baggage debacle often seen now a days. I should add though that easyJet in my experience have always handled it very well in the terminal prior to boarding meaning it's marginally easier for the crew to manage in the cabin. And I say marginally for any easy crew on the forum.