The Movistar management are pointing to two factors in the decision, firstly not wanting to put too much pressure on young Quintana to better on last year's podium-spot and secondly that Valverde has earned another shot at the TdF.
"Personally, I don't think taking Nairo to the Tour with his age, plus the pressure of improving last year's result, is interesting for his future. I prefer to keep him growing into the formation period he's still in and let him know the Giro, because we think it's an extremely interesting race for him to progress on so many aspects, and where he will enjoy full leadership in a Grand Tour for the first time. Naturally, Alejandro's presence into the squad also had its influence, having a solid leader for the Tour wit him. He has full backing from us - he's earned it. He already proved last year that he was perfectly capable reaching the final podium despite that mechanical."
This all makes sense of course but we already know that the decision is most likely influenced by two other factors as well, the wishes of the main sponsors have apparently played a part and there has been talk of an appearance fee from the Giro organizers as well. The truth behind those we will never know and it doesn't matter much anyway at this point. Fact of the matter is that this was really always going to be a win/win decision at least from a fans/Quintana standpoint. A TdF appearance would have been exciting since Nairo was so clearly the strongest Froome-competitor and looked like the most likely person to make the 2014 Tour into something more than a onesided Sky-fest. On the other hand the Giro (and Vuelta-combo) is so clearly the better race for the young Colombian, an opportunity to lead a team in a GT and with a very solid chance of actually winning on a course that suits him even better than the TdF course. From a purely sporting standpoint this is absolutely the best way but either decision would have been a winner for Nairo-fans. For players of our FSA DS game the thought of the Giro/Vuelta double is like christmas in February.
Add to that the fact that sending Valverde to the Giro would have been a complete cock-up. He's never had any interest in the race before and there's probably some lingering bad blood from the old CONI-ban thing so having him slogging around for three weeks probably wouldn't have made anyone happy. This way the Giro, which might be suffering something of a down-year in terms of turnout this year, will have one of the most popular riders in the world as their headline star which it probably desperately needed. With no major Italian superstars on the startline, the Giro is really going to have an uphill battle this year to capture fans imagination and a cool little Colombian surely helps.
As far as the Tour goes then Valverde is the safe way to go. He won't surprise anyone (anymore), he's a known quantity, he'll hang on impressively for most of the race. At some point he will cock up in some way or form that will put him out of contention for the top spots and then he will save face with a late stagewin or some impressive feat. We've been down this road before, Valverde is a truly competent stageracer but the Tour is just one size above his capacity with current restrictions in place (and most likely even without the restrictions).
Perhaps the most interesting part about having Valverde at the TdF is what opportunities it opens up for others incase he flops out early enough not to drag everyone down with him? There are some interesting second tier guys depending on who goes as support where, both Benat Intxausti and Igor Anton are riders we'd all prefer to see riding their own chances at this point aren't they? Either way, the Valverde-bet probably makes for a solid but unspectacular TdF appearance for Movistar with lots of ammo left in the gun for a dominant Vuelta performance. Movistar have a very real chance of winning two GTs this year, just not the Tour de France.