Why can't the orthogonal PN codes help in the near-far effect?
the orthogonal PN sequences used in CDMA be able to handle this?


Orthogonal PN sequences could. 

In reality, you don't get these – frequency and timing offsets, and trade-offs during the design phase mean that you only get approximately orthogonal sequences.

So, even if the cross-correlation between what TX1 and TX2 transmits is say -50 dB, but TX2 is 53 dB stronger at the receiver than TX1, then TX1's energy after correlation with the matching PN sequence is still only half of that of TX2's interference.