Basically, because there is no read ahead. What I mean by it is that Voyagers were never designed to make measurements of particle density, their magnetic charge, temperature and velocity with any instrument ahead of its path or even around it, so the spacecraft would first have to get there, do measurements in situ, and not detect any significant change for a long time so we could say with any degree of certainty that the last change detected was indeed due to crossing over the heliopause, where the interstellar medium pressure prevails over the solar wind of the Solar system.
But that is not all there is. You see, the Voyager's plasma instrument stopped working around the time it flew by last planets back in 1980, so any of the measurements of the pressure of the interstellar space on our heliosphere could only be inferred indirectly through measuring magnetic field, or more precisely measuring magnetic orientation of the extremely tenuous particle density medium (we're talking of up to a couple of particles in a cubic centimeter), as has Mark Adler already answered in one of his previous answers.
To put this into a perhaps easier to understand analogy, imagine you're on your bicycle pedalling through a fog. You can't see ahead at all, because we've tied your eyes so you'll be our Voyager 1 trying to detect the heliopause, or in our case, where the city fog clears and you've entered a different, countryside fog. You have never before been to the countryside, so you're not exactly sure how it will feel like getting there. You've been doing fairly good for a while, sticking your tongue out and detecting humid air, got a fair impression of its temperature, velocity, and so on. You're fairly confident that if you entered a different fog, you'd be able to tell that by some of your sensors. Its temperature might change, or the density, velocity, wind direction, and so on. But to make matters worse, just as you started approaching the city suburbs, your tongue went all numb, so all you were left to use is your nose, and hope the rural countryside fog will smell slightly differently than the city one does. You've detected some change, or at least you thought you might have, but did you leave the city limits already, or did you just pass a rather stinky dumpster? You press on, and for a while you don't smell anything. Is that it? You press on. After a long while, you realize that the smell didn't change at all, so you start believing you're already at the countryside. But can you be sure?
This is about why it's not easy to determine when exactly did (if at all) Voyager 1 leave our Solar system, and why NASA believes this time it really did back in August 2012. They didn't detect any significant change in magnetic orientation of the medium the probe is going through now in a good year. Now, I've again said it, didn't I? I've said if it at all did. Why?
Well, you see even though I personally have no doubt that it did leave our Solar system and entered the interstellar medium, the area of our Sun's influence isn't constant and this area would inflate and deflate like a balloon due to solar cycles, which is about 11 years long periodic change in Sun's activity. Solar maximum when the Sun is the most active, and Solar minimum when its activity subsides. Now, Voyager 1's velocity relative to the Sun is roughly 17.26 km/s, but the fastest solar winds can reach velocity of 750 km/s, which is faster than the solar escape velocity of 618 km/s. We could say, in a sense, that our Solar system is breathing, the Sun is inhaling and exhaling. Increased activity produces faster solar winds and they reach escape velocity, inflating the heliosphere. And there's the unpredictability of the density of the interstellar medium as our Solar system moves through it. We know it's not uniformly dense, and we call areas with greater density our local interstellar clouds:
Any change in density our Solar system moves through could also decrease or increase the influence area of our Sun. The truth though is that we don't know with what speed these heliosphere inflations and deflations happen, how much they vary in size, or how frequently the interstellar medium density fluctuates, so it's hard to predict if Voyager 1 could eventually be caught up again by the influence region of our Sun and overtaken by the fastest of solar winds, having it go through it all over again. But this is another reason why it is so hard to figure it out if Voyager 1 left the Solar system. The heliosphere isn't a static point in space. So this question isn't easily answerable with any of the options you gave in your question. In fact, I would lean towards saying neither are the most responsible for why it is difficult and there are other factors we need to consider, such as what sensors Voyager 1 still has left to its disposal, and dynamics of Solar and interstellar weather.