If you want a real explanation, this is how defensive wars against an overwhelming opponent are fought. Iran knows that they can't build an iron-clad air defense perimeter, there still isn't a reliable answer against stealth aircraft and cruise missiles. They never had a chance of shooting down every plane that enters their airspace, and that isn't their goal.

Instead, they will fight this war by absorbing blow after blow, hiding their capabilities and striking back when it is advantageous.

All Iran needs to do to win is:

1) Outlast the US air campaign - note this only requires protecting enough of their defensive capabilities to remain difficult. It does not require shooting down every US aircraft that enters their borders. It does not require shooting down most aircraft that enter their borders.

2) Prevent free shipping in the Strait of Hormuz.

That's it. They just need to apply economic pain as domestic and international opposition to the unprovoked attack grows.

Air defense is not static. Even fixed launchers can be moved, and reacting to how your enemy is operating is an important part of air defense tactics. The famous F-117 shootdown happened because the air defense operators carefully planned around how the US was using its aircraft. If most Iranian air defenses were destroyed in the first few days, it'd make more sense for them to hold whatever was still available for the sort of situation where they had much higher chances of scoring a kill than just throwing it out there to get destroyed immediately and accomplish nothing.

~15/16 MQ-9 Reapers have been shot down inside Iran. Not jets but still combat(strike and reconnaissance) aircraft.

I didn't downvote, but your post sounds like you're implying some kind of tomfoolery, deception, or other hidden reasons. There are very likely none, it just takes time to adapt to a specific enemy, probability slowly increases while you get more attempts, and then after some time (t) the first shootdown is "properly" successful. And note how this was preceded by that half-successful shootdown where the plane made an emergency landing. And they shot down drones.

You sound like they roll an antiaircraft cannon out of the hangar and immediately successfully down a plane. That's not how that works. The AA was probably there from the beginning, just not successful.

Because it obviously doesn't have the capability. Similar to how the US has no capability to "win" from the air only.

We don't know what downed it yet, so it's hard to say. Iran is hiding and rationing their offensive munitions, we know that, so it's not surprising when the number of drone and missile attacks spikes after weeks of bombing. That's part of the plan. But the ability to take down a US fighter jet is not something they are rationing- it's likely at the edge of their capabilities and they got lucky. If they could be knocking down more, they would be.