To see why, start by looking at some heal-per-minute (HPM) estimates from the invaluable TreeCalcs spreadsheet on Elitist Jerks.
- Lifebloom: 18.94 (assuming a rolling 3-stack)
- Swiftmend: 9.94
- Wild growth: 9.67
- Rejuv: 5.53
- HT: 4.96
- Nourish: 4.92
- Regrowth: 2.11
These values assume that there is no overhealing. For progression content, if you can afford to let people drop to 80-90% before healing them, then this is pretty realistic. The main exception is that lifebloom on the tank will probably do a lot of overhealing, but you really need to keep that lifebloom stack rolling no matter what. If you let it drop, you don't just lose the healing from it, but also the mana regeneration.
Other things equal, you want to cast high HPM spells. At the top of the list, keep lifebloom up and cast wild growth and swiftmend whenever they are off cooldown. At the bottom of the HPM list is regrowth. Regrowth is just abysmal for sustainable throughput, so save it for emergencies or for omen of clarity. Overall, the extremes of the HPM list give us the usual conventional wisdom for sustainable throughput.
The spells in the middle are more interesting. They all have roughly the same HPM. Nourish and healing touch are nearly identical, and rejuvenate is a little bit higher. This similarity in HPM leads to a counterintuitive result: for sustained throughput, it doesn't matter very much which of these three spells you use! The only thing to be aware of is that if you cast the mana-heavy ones, you need to cast them less frequently. Thus, the more you cast rejuvenate and healing touch, the more dead time you need to sit still and let your mana regenerate.
If you are tank healing, then the thing to do is keep rejuvenate up and then cast healing touch every once in a while. How often you should cast healing touch depends on how much mana regen you have. This is a counterintuitive approach, because most druid bloggers recommend constantly spamming nourish and then casting an occasional healing touch whenever you can. However, I don't see the advantage of doing so. You do just as much healing and use just as much mana, but by casting healing touch you are able to move around between casts.
If you are raid healing, then matters are simpler: just cast rejuvenate. Rejuvenate has better HPM than any other option, and it's an instant cast, so nourish and healing touch have literally no advantage. The only time you'd want to cast something other than rejuvenate on the raid is if you need to heal someone that already has rejuvenate on them. All these rejuvenates are expensive, but like with tank healing, simply insert enough dead time that your mana doesn't drain too fast.
Both rotations have a lot of dead time in them where you just sit there twiddling your thumbs and watching the pretty graphics. How much dead time is enough? It's a matter of theorycrafting, and I don't know. As a rough ball park, though, I believe it should be substantial, something like 40% of the time. Instead of nourish spam, which you can do indefinitely, you cast spells that are roughly twice as expensive, roughly half as often.
I know this dead time is counterintuitive. Dps casters must Always Be Casting to get their highest dps, but that's only true when they are in a position of never running out of mana. Also, all the blogging restoration druids are recommending never-ending nourish spam whenever your mana is tight. Perhaps their intuition is that nourish is "free", because while you cast it you regenerate enough mana to cover the cost. However, you still have to pay the cost of the spell. If you instead don't cast, for the same amount of time, you get to cast rejuvenate or healing touch sooner, and this makes up for the lost healing from nourish.
If Blizzard wanted to change this situation, they could make nourish do much more healing and thus have better HPM than healing touch and rejuvenate. Then what we'd cast nourish as much as possible and only use the more expensive spells because we become GCD locked. However, honestly, I kinda like the idea of having some free time to think and move around. It's a nice change of pace. It makes healing more deliberate and less spammy.