Captain Cannon also is the worst pick for long range fights. A normal Cactus will take him out long before he can even dream about going to win that fight.
And if you compare 2 characters you actually have to compare the whole characters, not just cherrypicking things.
P. Cactus got 150 HP, while Captain Cannon only got 125. That alone should tell you that P. Cactus is made to go closer.
And 150 HP is a lot. With the health upgrade (160 HP), P. Cactus can even survive an Imp Punt.
If you've got an Imp problem in your garden, you call Petrified Cactus. After that they will only dare to approach in their mechs.