what could have been
the shuttle was designed to be taken to near space by a lifting body, a sort of super jumbo, not these great big fire works they now have, light and forget.
the posh name was HTHL, horezontal take off , horizontal landing.
so the tiles being brittle did not matter so much, as they were protected by the slow take off speed and gentle handeling, and very light was an advantage,
Then cost cuts come in, and hay presto, you make the typical engineering decision, the best of a bad job.
The tiles have never been covered or painted. The external tank was origionaly painted, but bits flaked off even then. The major problem seems to be Ice, formed around the tanks, which has always been a problem using super cool stuff like LOX and H2. look back at the apollo launch films, you will see ice every where.