The technical reason is because you cannot tech a throw if you cannot perform a throw at that frame.
If you are being hit out of an attack by a throw, you can't tech it; That's a counter throw.
If you are in the jump start up of your double jump, you cannot input a throw, so you can't tech.
If you are crouching and you press throw, you get C.LP or C.LK, you can't throw, so you can't tech either.
The gameplay balance reason is so that opponents can mix you up by choosing between low or a throw.
Stand up to tech a throw = risk getting hit low.
Crouch to block a low = risk getting thrown.
You can let go of down to tech a thrown if you are fast enough, but if you're trying to do it on reaction like "WHOA! I just got thrown I'm going to try and tech that!" You are way too late.
You have to know the low/throw is coming and instead do something like, block low, pause, stand up, throw tech to try and cover both options quickly.
If you could tech while crouching, grounded mix up would be too easy to deal with and it would really hurt characters with no overheads that rely on throws (Double)
Someone made a video about dealing with tight low throw set ups here:
If you have any other questions let us know.
