OpenTable only does reservations when facing the diner, but when facing the restaurant they do reservations, CRM, payments, table tracking, and analytics.
Whoever wants to replace OpenTable needs to replace all of this at once. Not only that, they need to convince a restaurant to ditch practically their entire IT stack, retrain all of their employees in the new system, and do so without service or quality interruption.
OpenTable has positioned themselves to be about as sticky as you can imagine a software business to be. They've locked in their vendors hard. Evil? Probably. Very effective? Hell yeah.
It's not as if no one's challenged OpenTable before - there are in fact a number of restaurant management startups out there, but none can even dent OpenTable's dominance due to the lock-in. Google couldn't take out OpenTable even if they tried, not without burning a pile of cash taller than this current purchase price.