Ravenclaw is for all the smart wizards. Presumably, anyone who ended up as a Professor must have been pretty smart. So why are there Professors not from Ravenclaw?
Hufflepuffs are quite resourceful and reliable, very good qualities to have for a teacher trying to teach a difficult subject.
Slytherins are quite tricky and manipulative, and knowing how to use that to slip knowledge into a student's mind would be quite like them.
Gryffindors are courageous, and what sort of courage it must take to teach a classroom full of spell-potent young wizards.
In short: There are more qualities to being a teacher than just "knowing everything".
And, more to the point, the most scholarly student of them all in the series, Hermione Granger, was in house Gryffindor. So clearly there's more to being chosen for Ravenclaw than simply 'You are the smartest'.
Intelligence isn't limited to those who are in Ravenclaw, and it isn't the only characteristic that the Sorting Hat looks for when assigning students to Houses. It's perfectly possible to be incredibly intelligent - more than enough to potentially be Sorted into Ravenclaw - but demonstrate something else that ultimately means you would be better suited in another of the four Houses.
Just look at Hermione, who was the smartest student in her year at Hogwarts, but was Sorted into Gryffindor because she demonstrated potential for great bravery, which evidently outweighed her intelligence.
There are two or three assumptions here that need to be debunked. Firstly, the idea that teachers have to be the smartest - in fact, if the wizarding world is anything like the real world, many of the smartest and brightest may well have shunned teaching for other pursuits. Secondly, the idea that intelligence is inherently a bookish thing, and that it is the most important trait of a teacher (really it's just one of a number that teachers need). And thirdly, the idea that the Houses and their students are all so obviously not alike.
The beauty of the different House traits is that they are all so broad as to mean that any one individual could show characteristics of all 4 of them. Just look at Severus Snape (perhaps one of the more complicated characters in the novels): he is tricky and manipulative, a born Slytherin who deceived Voldemort (and the protagonists). Yet he was also "the bravest wizard I have ever known" (according to Harry) due to his working as a spy against Voldemort (Gryffindor's primary trait). His brilliance at school in potions (as demonstrated by the Half-blood prince book he left) show he had the ingenuity characteristic of a Ravenclaw. And his loyalty to Dumbledore and to his affections for Lily, a loyalty and dedication that lasted right to the end, show he had a bit of Hufflepuff in him too.
And let's not forget the very real possibility that people change, with time. Snape again, may be a good example of this. What House would he have been sorted in, if he was Sorted around the time of the 7th book, instead of as a child?
According to Pottermore, Professor McGonagall was the premier Transfiguration academic of her time (and before her, we of course have Albus Dumbledore's academic achievements). Both were in Gryffindor, and both - leaving aside all OTHER considerations - were clearly more academically qualified to teach Transfiguration than any competitor, from Ravenclaw or otherwise.
This question begins with an incorrect assumption "Presumably, anyone who ended up as a Professor must have been pretty smart." This is easily dismissed by reviewing Professor of Divination, Sybill Trelawney, She clearly does not fit in the category "pretty smart". Another large character/professor may also be identified, as a tough fit for "pretty smart", who I will leave unnamed for those who have not progressed through the entire series.
As "pretty smart" is not a requirement for professorship, a house has little to do with becoming a professor. Not to mention the facts presented in other answers concerning the distribution of Intelligence across houses.
This is a fairly common assumption about Ravenclaws, borderline a stereotype.
Here's what Pottermore Sorting Analysis has to say about Ravenclaw:
That's the reason Hermione is not a Ravenclaw: She is not open-minded, creative, or tolerant, although she is well-rounded, all Ravenclaw traits.
First of all, just because somebody is from another house doesn't mean they aren't smart (ex: Hermione, and even Harry has relatively good grades through the series.)
Secondly, most professors at Hogwarts don't seem to be overly well rounded (See quote above.) Could you see McGonagall being the potions master and Snape being the Transfiguration teacher? Sure, they can probably do it well, but they likely haven't completely mastered every single subject as much as they have mastered their specialty.
After a thorough search of the JKR website, I found nothing on how a teacher of Hogwarts is qualified, although I suspect that they need only a Outstanding NEWT in the specific subject that they want to be qualified to teach.
Third, certain traits from other houses might overpower Ravenclaw traits. Let's use McGonagall. According to Pottermore, McGonagall was a Hatstall and So was Flitwick, both between Gryffindor and Ravenclaw. While they both possess all the Ravenclaw and all the Gryffindor traits, McGonagall has a higher ratio of Gryffindor traits and Flitwick had a higher ratio of Ravenclaw traits, although they had both.