From the biological standpoint, during sleep and dreams part of the brain is shut off(dorsolateral prefrontal cortex?). Specifically, it is one of the logic centers of the brain. It has been theorized that most of us don't become lucid every night due to this part being inactive. We can't logically figure out that flying green cows aren't normal and become lucid.

Reading, math, etc. are all logical things. You must use logic, even if on a low level to read and interpret letters into words into ideas. If the logic center is out of commission, then it stands to reason your sleeping brain wouldn't be able to support static logical characters once you stop paying attention to them.

In short, a possible reason is your inability to think logically in a dream. Natural lucid dreamers probably don't have their logical thought processes inhibited as much.

Also, placebo is definitely a factor. So much of what we can and can't do in a lucid dream is based on our expectations, it stands to reason that some of those who are capable of reading normally aren't able to because they are prejudiced to think otherwise.