Probably technology but not in the way you might think. I think it is quite possible that breakthroughs in certain fields simple lead to the elimination of the need to build ships as huge as the Kelvin. Lets assume that we need 800 people to operate a Kelvin class ship which performs a deep space surveilance role. Let's assume about approx. the half to two third of the crew is needed to maintain the ship and its facilities because of her relativly low tech (which isn't unreasonable considering her industrial style) and assume that only about a third are actuall mission specialists and now let's assume that there where some really cool breakthroughs, for instance:I'd like to hear what you think the limiting factors - the Panama and Suez Canal analogs - are, because I can't think of anything equivalent. If the PoD was as late as the Kelvin's destruction, then SF had the means to construct Kelvin/neo-E sized ships all along, and there was some other limiting factor.
- Transporters are much more practical to the point where the whole flight deck is beeing reduced to about four shuttles. (Cut out the huge flightdeck of the Kelvin).
- Splitting the nacelles into two and other advancements lead to the whole propulsion system beeing not only far smaller but also a lot more maintance-friendly and efficient. (Cut out the need for half the crew including accomodation, provision etc along with an unkown size of the reactor and fuel supplies.)
- A really cool Phaser is developed, a breakthrough in accuracy, power output etc yet you can only mount so much because of reactor restraints so mounting more than a certain number makes no sense, certainly the large clusters of whatever the Kelvin is firing alongside old style phasers are no longer needed. etc.
All those things which can reasonably be assumed could lead to ships like the Enil which are able to perform the same missions much better, bring the same number of specialists to wherever they are needed and still perform better and more efficiently in a military role.
But but but why not build a ship like the Kelvin, mount it with the same technology, with four reactors, twenty new phasers and whatnot and watch the admirality faping to their newest male ego booster, you say? Because in the geneverse it wasn't needed, that simple, all you would achieve is starting an arms race with the klingons which COULD be exactly what happened in the abrahmsverse, where Starfleet might not be reacting to the single Kelvin incedent, but to the Klingons who reacted to Neros big ship (and suffered much more, depending on if you count deleted or cut out planned scenes)