The PA-11 was designed and delivered to the public rather quickly but it was also being considered as something of an interim model by Piper and thus, its production was somewhat limited for two reasons: In the first place by then there were several thousand J-3 still flying, and many of these aircraft - those with aluminium wing ribs - could easily be upgraded to PA-11 standards by means of rather inexpensive kits that made a lot more sense for J-3 owners than buying an altogether new plane.
Marcelo Alderisio shows how easy is to clean the fuel line by draining the filters.
And the second reason was that Piper was already working on the PA-18, a superficially similar but bigger, heavier, much more powerful and capable aircraft. Ironically, kits for upgrading the PA-11 to PA-18 were also developed. These included flaps, more powerful engines and a second fuel tank, among the most noticeable items.
Some essentials for staring a PA-11, like a fire extinguisher and chocks; you can see at the bottom the wind or aeolic alternator for the electrical system of the plane.