AFAIK the IBEX isn't a photographic telescope, it develops its images through energetic neutral atoms and not photons. It takes over 10 hours for light to get from the sun to the heliosphere, so to retrieve particles that are travelling at sub-light velocities and are moving into the solar wind must take considerable time alone, and then it requires building an image from the particle densities retrieved.
Simply put, unless you ask a NASA scientist exactly how and why, the answer is and likely always will be; it just does.
You don't retrieve those particles; an admittedly absurd extension of your reasoning would imply that it takes 200 million years to image a quasar, because that's how long it takes to retrieve the photons that form your image.
What we're seeing here is both faint and enormous. Both characteristics make it hard to detect, and it isn't bright enough to capture without a long exposure, plus you can't look at it during the day, and you can't see the side beyond the sun... all characteristics that lead to longer imaging times.
Simply put, unless you ask a NASA scientist exactly how and why, the answer is and likely always will be; it just does.