Because obviously having all one class can easily break an unprepared team, as we have said, there are advantages for that kind of surprise or unbalance if you know what the other team consists of. I could find a few textbook words to describe it, but the most common might be, I think, "cheap".
Sorry for thinking you were referring to me before in any way.
Junta - several clan members noticed at once, remember? That was weeks ago. Yes, it was at a point of wanting to do 4 engineers (shockingly to build each others stuff in a smaller game), but has cropped up since then (like I mentioned). In big games, 13 players is a lot and you might well see the need for 4 or more of a class, even the ones you chose to limit.
Of course, in a clan game, 2 class limits on 6 team sides make much more sense - since this is 33% - taking that number into 13 players gives us 4 minimum, and given the 13 players means many classes are likely to be played more then once, even more then twice, 4 might not be so uncommon to reach.
But in any case, I've put forward my arguments against this

In addition - surely if people hadn't noticed, it's a bad thing - they'll be quite surprised to see it in place for the very few times it occurs - and that if it was a more serious limit of 2, you'd actually have people complaining actively since they would hit it more often?
I mean, you could sidle in class limits of 10 each, who would notice them? Saying then, that class limits are a good thing because no one noticed them being in effect is kind of against the point of having them in the first place - they are there to actively stop people playing a class, therefore, they should be the most noticeable of limits, and if not, then either the number is too low (and so no one would ever have that many of that class), or not necessary (no one ever gets to that number thus it isn't required for the very minor amount of cases - in the lowest of digits - that an admin can't just come in and sort out, or in fact, encourage if it is a fun set of amount to play)
I also take issue with the fact scouts are not class limited, despite being, for all intents and purposes, more of a utility class then the engineer and sniper (who have actual massive point scores, and team contributions, despite being loners) scouts are just loners who jump around a lot, and are a bit more "obvious" I'd have thought to limit. I'm generalising here, but you can find a lot of the time if you find a team half-full of scouts, that team will lose, and lose badly, especially if on defence.
Oh, unless the point is to reward the one strategy of "scout rush" but not allow for the "engineer defence" or the "sniper defence"...I'm very confused on that point.