There are a number of possibility...
Firstly, no reason... just prefer to keep low profile...
Secondly, she feel ashame of her guy... So she don't wants the
whole world to know...
Thirdly, out of shyness...
Forth, she don't feel that it's the right time to tells
other...
Actually, there are alot more reason and the percentage of getting
the each of these reason is the same.
Don't think to much, one solution, ASK.