Mormon 9:32-33 points out that the plates were not large enough to be written in Hebrew:
And now, behold, we have written this record according to our knowledge, in the characters which are called among us the reformed Egyptian, being handed down and altered by us, according to our manner of Speech. And if our plates had been sufficiently large we should have written in Hebrew; but the Hebrew hath been altered by us also; and if we could have written in Hebrew, behold, ye would have had no imperfection in our record
The Hebrew language, like English, is spelled out letter by letter whereas Egyptian (and shorter versions of Egyptian) can take up much less space as their symbols can convey entire words or phrases.
Joseph Smith did not learn Hebrew until after the publication of the Book of Mormon, and Egyptian was only recently being understood at the time, so how would he know Hebrew would take up more space than Egyptian?