As for the two "th" sounds... well, there are a ton of consonant-like sounds that didn't make it into lojban. We can't get them all in there. I seem to remember hearing an argument against [th]e that went something like "many languages don't have that sound and it is extremely difficult to pronounce for people who haven't grown up with it".
I think a similar argument can be made against many of the omitted vowels. Also, many of the vowels in English are pretty close to others. My mother can barely tell the "a" in "bat" and the "e" in "bet". It makes it easier to tell words apart, which is part of lojban's aim.