There's usually only one reason a television show gets cancelled: money. What determines the magic number separating renewal and cancellation are two things: ratings and age. Take these things into account and you'll understand my answer to this rhetorical question:
@wilw Monday viewers #Alphas=1.39Mil, #warehouse13=1.82Mil, #Eureka=1.87Mil. Yet WH13=renewed and Eureka canceled. Does this make sense?
My answer: yes, it does.
Five seasons is the magic number for syndication deals, but that doesn't matter to the network (SyFy, AMC, USA, etc) because they don't own the show and don't get the syndication money. But it's also a magic number for contract negotiations. Although I'm not an expert on these things by any means, networks will lock up talent with contract options that allow the show to be renewed without breaking the bank at least until the fifth season.
Everyone is going to want a pay raise if their show reaches that age. And it's hardly an unreasonable demand. Any show that reaches five years can and should be deemed a success, and the people responsible for that success should share in it with the network and studio. These negotiations can be messy at times. Rob Lowe left The West Wing during its prime, and Claudia Christian leftBabylon 5 because the raises people were due weren't handled the right way.
Any show hitting its fifth season is going to cost more to produce, perhaps by substantial amounts with a large cast or a critically acclaimed show. But that same show isn't likely to see an increase in ratings at the same time.
Ratings determine how much a network can charge for commercials, which in turn determines how much a studio will be paid for producing it.
What all of that means is that the longer a show is on the air without a regular increase in ratings, the less profitable it is for the network. Shows like NCIS andTwo and a Half Men can push a decade or more without any increase in ratings because they were more profitable to begin with. If Eureka had 20 million viewers, it'd have no problem staying on the air for another five years or more.
So the answer to the question above should now make sense. Eureka got canceled and Warehouse 13 did not, even with equal ratings, because 13 has only been on the air for three seasons, while Eureka will be five and done.