If we think of the universe as 4d space-time then it is just a geometry, unmoving, unchanging. We might ask why this geometry, but a starting question is, why is the time axis so different to the space axes?
Well, relativity tends to deal with how the two relate to each other, but why is there this strong dependency in the time direction?
An answer may be that the universe geometry in 4d is radial around a point, in 2d it might look like so:
The radial lines represent information, since there is a strong correlation along the length of the line, you might say that the direction of the lines (which for a small neighbourhood are roughly parallel) represents the time direction, and the other 3 directions are space directions.
Since the correlation is in this direction, it remains to decide which direction is forwards in time, this is simply the direction to higher entropy.
So I am suggesting that the time dimension is simply the one that happens to be the direction of the lines in this radial 4d shape. It explains why the universe appears to be expanding, is consistent with everywhere being equidistant from the 'big bang', and deals with why there is nothing before the big bang (because that is the centre of the volume), it is approximately the same reason that there is nothing north of the north pole, just a geometry issue with our way of parametising time.
Accelerations or decelerations of the expansion could be explained as changes in the density or roughness of these lines, effectively changing how much time is perceived to pass per unit distance along the line.
OK, so if we accept such an explanation of why we perceive a 3d universe changing through time, then the next question is: why is the universe geometry shaped like that and not some other shape?
We can't ask why there is matter in the universe, because matter is just energy (movement) which is just a correlation of information in the time and space direction. All that exists in this universe is just correlated information.
We can't ask what created this correlated information as we have removed the idea of time and cause and effect, so all we can really ask is: why is the information correlated as it is?
Which can be rephrased as: why are there these constraints on this information?
I can only consider this question speculatively... I am fairly certain that the only sensible answer is that there are ultimately no constraints on the information. In other words, each apparent constraint is an emergent constraint from the action of multiple, smaller level, less constrained information.
This idea can be seen right through physics, from Feynman's path-integral descriptions of particles, to entropic forces which explain elastic forces and probably gravity.
So at the highest resolution there are no constraints and therefore there is nothing that needs further explanation.
i.e. we are living by the high level laws that derive from infinite information acting without rules. Even logic and consistency needn't apply at the lowest levels, it is just that logic and consistency is more persistent, an invariant state or attractor of the mass of possible illogic.