Because people Google the skill description and by default it's 180s until the WHM gets the passive trait that reduces it.
This more than anything else. There's a lot to take into account with AST MP management and when you do learn all of that it's a lot easier to be in control. AST shouldn't really have a hard time with MP.
In terms of LA usage, definitely don't just use it at a certain MP level. If you think a pull or a boss is almost over just hold off on it and you'll be back at high MP levels before the next one. If you're at the start of a big pull/boss phase where you know you'll be pumping out a lot of heals, sometimes you may want to pop LA at the start so you have a good amount of mana when you're done. However this too is situational. If your group has a lot of dps and that big mob pack won't take long, you don't need the LA. If the next phase of the boss fight allows you some downtime, you don't need that LA, etc. Know the pace of the dungeon/fight and trust your own MP pool, because if you waste LA when you don't need it then you won't have it when you do.
Then there's also Lightspeed and Ewer. Ewer is just a bonus and usually not needed, but if you do get it then you can plan around that and it just makes things better (and Spear I never use on myself and you should not need to either - the tanks and several dps classes actually benefit quite a bit from this, so give it to them). Lightspeed can be useful for a few things - some fights if you need more mobility, you'll want it then, but with a short enough CD you can usually use it at the start of a fight and have it back up for that movement heavy phase. Or if you know you'll need to pump out a lot of AoE healing in a certain part of a fight, or spam heal the tank, then pop it then and take advantage of cheaper Benefic II's (where I otherwise would not have used that many Benefic II's). It's a great way to save mana, however, so that's my primary usage of it and outside of needing it for a particular phase, I try to use it as often as possible for conservation purposes.
But more than that you need to be more efficient in your regular skill usage. Keeping up Diurnal Aspected Benefic on your tank almost all the time (as long as he's taking constant damage) is a great way to save mana. No it's not as amazing as the WHM Regen, but it's the best we've got. Outside of that, I try to keep up only Benefic I spam on the tank. Benefic II is reserved for the occasion when he dips a bit further, but once he's back in safe territory I go back to my Benefic I spam. If he needs more healing than that (i.e. in big trash pulls), then double HoT the tank instead of spamming Benefic II for that duration (double HoT + Benefic I spam is more mana efficient long term than Benefic II spam. And like I just said if he temporarily dips low, use an oGCD ED in between casts and one or two Benefic II's if you need them when that's on CD, otherwise back to the Benefic I spam). If I really need a bit of Benefic II spam, Lightspeed first. Rarely should you need to use Benefic II outside of these circumstances. It's incredibly mana inefficient so avoid it at all costs.
Diurnal Aspected Helios is great for AoE healing and what I use most of the time. If people only took a small hit and won't take more damage immediately, often I'll let them regen naturally if it's so tiny - you don't need Helios for that. If one person takes a bigger hit I'll heal them up individually. Helios really I only use as a follow up to AH when there's a lot of damage going out.
And let your HoTs actually tick. Don't top people off if they have a HoT rolling and don't top them off unless your Benefic I will actually be of use (i.e. don't over heal). I've seen people say don't top them off in general, but really you should have enough mana to keep people high enough. If you're so strapped for mana that you can't afford to actually throw out one Benefic I to your dps then you're doing something wrong.