Ok this is my opinion and I dont know right off hand scripture to back it all up, though if pressed and when I have more time ( and its not 1 am LOL) I could more closely explain why I believe this way and back it up with scripture.
I believe (ethnic) Israel was punished on Mount Sinai, because they asked not to hear God again.
Mount Sinai is also likened unto Hagar( the bondwoman), implication ( to me anyways) that Sarah( Abrahams wife) is likened unto the New Jerusalem. This is in Gal 4 I believe.
I believe the "light" (truth of the gospel) and the place where we are today spiritually- knowing that fleshly circumcision really foreshadowed circumcision of the heart, the cutting off of the "flesh", the old man dying, the new man being risen with Christ etc etc...could all have been done back then, had they only had faith.
scripture says " how many times would I have gathered them as a hen gathers her chicks, buy you would not.

ack, that makes me really sad. But anyways..
The "Truth" was veiled, when Moses spoke. I believe this is spoke about in Gal 3.
It wasnt spoken of with "plainess of speech" like Paul did.
You know Pauls mystery? How he revealed a mystery that was kept hid from the beginning of time?
I believe the mystery revealing is that he explains what the law and the prohets told, scripture that was written long ago- but they ( ethnic Israel) didnt understand it.
Because they were in darkness. They had no understanding. They didnt see the things that are explained in Hebrews. Hebrews is a really deep book, if you read it with the understanding that (ethnic) Israel knew the law very well, but didnt understand that it foreshadowed the coming of Christ, and that he would be the high priest, etc etc..
You'll see how the writer goes back time and time again into the old testament to use OT scripture to show them that THIS is what it really meant.
Ok anyways, to answer your question.
"Plainness of speech"(explaining Jesus' function) could not be used until Jesus had died and rose again.
The old covenant still stood, and the new couldnt be implimented ( Im sure i lost ya right there LOL, most dispensationalists dont believe we are in the new covanent) until the "testator" had died.
The Comforter( by whom truth is revealed) could not come till He had died.
The entire time Jesus was on the earth he spoke in parables.
He asked the pharisees if they could understand, and he repeatedly asked His disciples if they understood yet.
I imagine Jesus standing there telling a parable , knowing He couldnt come right out and just spill the beans saying " GET IT YET?? HUH? HUH? HUH???SEE what I meeeaann????" <wink , wink>
The reason is like I said- he could not speak "openly" because the old covanant was still in effect.The old covenant kept Israel in darkness.