I apologise for making a few assumptions here, but you don't say where you live and it matters with peas. If bonny are Sugar Bon, then they will grow about 15 inches tall - usually! Looking at the time and size you transplanted them, would indicate that you started them a little late for most parts of the US. I am guessing here. Peas don't like heat and will stop growing when temperatures hit the 80s or higher and stay there for a couple of weeks. Maybe this is what is happening?

But if it is still cool where you live, say Washington or perhaps northern Maine etc, then you might well see further growth. If they are producing peas for you, it doesn't really matter how tall the plants are.

Latitude plays a part in pea height, the more northern areas grow taller peas generally. Maybe you are in the South and that is the maximum height this pea will grow for you anyway? Maybe sowing earlier would be possible to give the peas the best chance to grow and produce when the weather is still cool. Peas can be started earlier than beans because they can tolerate a light frost.

Were these Sugar Ann Snap peas sold by Bonnie Plants at many garden centers? If so, the Sugar Ann variety of snap pea is a short plant that produces pods quickly. Yours seems very short even for Sugar Ann, 18"-24" is more common. Perhaps hot, dry weather or transplanting slowed their growth. I've not tried transplanting them. They grow quickly from seed and I want many plants since each one only produces a handful of pods.

Most snap pea varieties get an initial flush of flowers that form pods and then may continue to flower and set pods at a slower pace if the early pods are picked before they are swollen with seeds and the weather stays mild, and they are well watered. Hot temps will shut the plants down and they will likely die. Sometimes shading the pot to keep the soil cooler or moving a pot into a shaded area can give you a bit more time with the plants. However, peas are a short-lived spring/early summer or fall plant for most of us, and they typically give a harvest for a couple of weeks.

Here is a link that might be useful: Bonnie Plants, Sugar Ann Snap Pea