There are several reasons for this, most of them practical.
I'm away a lot and not in a position to care for the caterpillars on a constant basis.
When the season began, I didn't have a lot of Swan Plants, due to a big clear out and repositioning of plants at the end of last season.
We're having a very hot, dry, Summer in Northland. We collect our own water from the rain and store it in a 9000 litre tank. 9000L isn't a lot (most people have a 20,000L tank) and when it doesn't rain (as it hasn't for 6 weeks or more) we must be sparing with our water usage. We can buy water, yes, but right now there's a long waiting list of those wanting to buy water and even the water source (a nearby river) is running very low.
Unfortunately my plants suffer in these conditions. Now that the season is in full swing, and conditions are dry, my swan plants are not in good condition and are quite aphid-ridden.
We are diverting water from the washing machine, to a separate tank for water to hose the garden with, but even then I have to be sparing with garden water.
Having said all that, despite the difficult conditions, over the December 2016 - February 2017 period I have successfully raised and released 36 Monarch butterflies. All were perfect and beautiful.