Probably because it is - although this is a desert type plant, coping well with dry conditions, when its newly planted, it does need watering deeply at planting time, and probably 3 or 4 more times in summer. In a pot, its more likely its dried out than in the ground, where its got a nice mulch to keep moisture in. I'm assuming there are drainage holes in that pot and it's not standing in something that keeps the water in; if this is not the case, it might be the opposite problem, too much water at the root.
The only other possibility, and this may not be relevant depending on exactly what you used, may be related to the soil you've used in the pot - was it soil you bought or soil from your garden/yard which you amended with compost? And this 'small amount of compost' you used, how was that produced? Using a hot aerobic method, so that its pathogen free, or a cold system? If it was a cold system and you used your own garden soil, there may be some pathogen in the soil that's affecting the plant. This wouldn't matter in open ground, but it does matter in a pot, where there is restriction of movement for bacteria and pathogens, so that they're more concentrated. It's a low risk, but still a risk.