Although nominally N-P-N a transistor is not at all symmetrical - the emitter is usually
very strongly doped n-type to reduce its internal resistance and increase the number
of charge carriers it can inject into the base. This makes the emitter-base junction
have a very low reverse-breakdown voltage (I believe rather similar to a zener junction,
very narrow depletion zone).
Conversely the base-collector junction is lightly doped and optimised for large
reverse-breakdown voltage as this is the normal state for it.