It is not known that solar flux has anything to do with the development of life; in fact, considering that life is known to have first developed in the ocean before land, where solar energy flux is actually lower than on the surface, it would seem to suggest there's not a direct correlation.
It's actually a well-known and confirmed fact that more energy means faster metabolism, and faster biological processes. It's true that it's not a strictly monotonic function – you see animals with faster metabolism at places with lower solar input, and of course you have deserts where the low precipitation have a stronger effect than energy; but nevertheless there is a clear correlation between the energy input and metabolism rate, in fact metabolism rate has a strict upper bound defined by the energy influx (you can't consume more energy that you have).
Of course, it's possible that photosynthesis was more efficient on Mars, it's possible that the energy influx contributed to other factors (such as precipitation patterns) which allowed for faster development of life, and it's also possible that the atmosphere back then kept more of the energy, etc. But the
expectation is exactly the opposite, in fact both of the factors that are correlated to the speed of biological processes – the energy and the abundance of water – are assumed to have been well below what's on Earth.
Speculation. The statement "not everywhere" = "probably not on Mars" is a logical fallacy.
If life is on less than half the planets that exist in the "habitable zone" (provided that the habitable zone matters), then life on Mars is less likely than 50% (panspermia excluded). Less than 50% means that probably there wasn't life there, and I believe the default value for that figure should be far lower than 50%.
And of course, the panspermia shifts the odds only if life on Earth came from Mars, because if it went the other way around, Mars would have been late for the party from the start.
We've investigated probably .0005% of the Martian surface up close. The equivalent of a housing subdivision.
No. We have investigated nearly 100% of the Martian atmosphere by spectral analysis, we've investigated the soil at several sites distributes far from each other, which might be isolated but give evidence for what the soil is over the entire planet after a simple extrapolation, and we've photographed what, the entire surface, with under a metre of resolution, we've also studied Martian meteorites that are supposed to tell us something about Martian past. "0.0005% of the Martian surface up close" doesn't mean we've investigated too little.
Sure, the evidence in the atmosphere is probably the first to go, and so is the visible evidence from orbit (unless you had a
civilization there, in which case some of the ruins would be evident from orbit even covered with soil), but any evidence left in the soil should remain there for a long period of time and if life was abundant it should be practically
everywhere. Now, how long is long is another matter, not to mention that you could write a book about the ways we could have missed it while it is there, but the fact remains that we haven't found any evidence yet, which means that the probability of life, especially complex, abundant and long-lived, has decreased (by a lot) since the time we began to look for that evidence.
A civilization of sentient beings on Mars is like a pre-human civilization on Earth – it's possible, there are ways it could have happened, and there are ways for the evidence to have remained hidden from us, but the safest bet is to assume there
wasn't one.
