There are a couple of things to keep in mind here (I hate to bust out fancy economics terms, but it seems that my econ class wasn't useless after all).
Most players aren't Homo economicus, as in they don't know what's the best for them. It's attractive to get the class you want to play in a lobby with 2 players that won't start all of the other lobbies are started (and probably until they're over to get these players back in the system). This is fine if you have plenty of time. However, if you are short on time, you'll be able to play less lobbies, or maybe none at all (and the goal of TF2C is, in essence, to play lobbies).
When people don't join a lobby, it's basically a market failure: the private marginal benefit is larger than the social marginal benefit. This means that although it's better for the player himself (which is arguable as explained above), there's a cost to the overall TF2C 'society'. One way to fix this would be to, you guessed it, decrease the number of lobbies (I could draw the graph, but I hate to admit that my econ class was useful beyond by exam).
Let's think of things in a mathematical way. Assume a finite number of people who exclusively join the top class they want to play. If there are no lobbies, then no one can play. If there are infinite lobbies, then people will spread out, since there is not an equal distribution in class popularity and since only two people can attain one slot. [Infinite is only to simplify things, since with finite players there are infinite lobbies with no one, therefor there's a finite number of lobbies for which there's this effect]. Thus, the lobbies will take longer to start. Now if there are less slots than people who want to join, then, by pigeon hole principle (look at me go with the fancy terms!), not everyone will be able to play. However, if they are willing to offclass a bit, then lobbies will go at full capacity.
The trick is to hit that sweat spot. Some people will have to play less they're second or third favorite class, but this is to the benefit of everyone else (I'll refrain from quoting spock). From a probability standpoint, people will be able to play their favorite class, but not necessarily always.
The trick is to dose the number of lobbies in terms of how many people are online. Obviously having a fixed limit is ridiculous. Hell, if the devs get around to having the TF2C servers automatically available for use, we can use those as a buffer in case there aren't enough lobbies (which was I believe one intended feature).
Finally, restricting the number of lobbies is almost invisible to people. People are way more inclined to complain that there are too many lobbies than that they are not able to play their main class. So this is a decent solution politically too.
Note: Please don't see any of this as condescending, I'm literally trying to as unbiased as possible. Except maybe for the following joke:
TL;DR: outnerded nerd