I can only make guesses here, but I'd think the reason Sam & Max will not be on Steam is because Telltale has its own digital distribution system - Telltale Now. The reason it's also on Gametap (which I'd argue is not the same kind of service as Telltale Now or Steam), and the reason Gametap is premiering it, is because Gametap co-produced it, which is probably a less blunt way of saying they helped finance it.
You make it sound like you know Valve offered Telltale an ideal agreement and a boatload of money (or, for that matter, that they even talked at all) and that they refused it not because of important underlying reasons but simply because they didn't know any better.
A Telltale representative is the only person who could tell us the nitty gritty details involved with selecting their partner, but that's unlikely, and, really, unnecessary, because the reasons given make enough sense on their own. Nothing I'm about to say is necessarily factual, but darn it, if you can speculate then I can too (

):
1) The way I see it, Valve is a competitor of Telltale. Telltale's trying to establish
themselves as
their own digital distribution system. It wouldn't make sense to give up control of their property to another company doing the exact same thing. From what I gather Telltale's trying to be as much a publisher as a developer. While for us consumers it would be convenient for there to be one single service for all digitally distributed games, a company has the right to sell their own games their own way without having to give up sales, creative control, or whatever compromises are inevitably made in such a contract.
2) Apparently funding was needed for the independently developed Sam & Max, so collaboration was necessary. According to Telltale they talked to a lot of partners before choosing Gametap, and I'd imagine Telltale would know more than you or I which deal would be best for them. Gametap is indeed a digital distribution service, but a totally different kind. They're a subscription service that mainly boasts itself as a kind of gaming library, with exclusive content (Sam & Max being the first) serving as added enticement for potential subscribers. The Gametap spokesman in that "Making of Sam & Max" interview described Gametap as an HBO with Sam & Max being their "Tales of the Crypt."
3) What appears to be the case is that in exchange for creative freedom and money for the game, as well as Telltale getting to retain control of their product, Gametap gets to showcase the game 15 days early. That seems very fair and makes a ton of sense in context. The fact the Gametap is not yet worldwide is unfortunate, but a two week delay is probably better than partnering with a company that mandates Sam & Max be a driving game (which may be an exaggeration but this was suggested in a Kevin Bruner interview).
4) We can't just assume that any theoretical contract between Valve and Telltale would have been the overall best situation for the game, for Telltale, and for the players.
So instead of equating the fact that Sam & Max is on Gametap two weeks before its full release with a slap in the face to Gabe Newell or the international gaming public, you should consider the possibility that Telltale had more on their minds than trying to personally piss you off when they were deciding who to join forces with for producing the games.
Of course, it's not like I can blame a lot of people who've complained about this. It's not every gamer's job to research the details of the publishing contracts for a game they want to buy - all they know is they're being screwed out of seeing a Sam & Max game for two weeks. In that light, knee-jerk reactions are expected. But I hope more people come to realize that the whole Gametap thing was probably for good reason.
And that's
my rant for the day. (Hey, what else is there to do as long as Mojo's down?)
EDIT: Ack, Linque beat me! And, unlike me, avoided being overwrought.