Four mains reasons:
1) Many dudes are really paranoid about privacy so they believe that if they write a review someone out there will know who they really are, and since most of those dudes have well-established social status they are afraid of jeopardazing it;
2) Many are what I call 'callous dudes" dude who think that if they wrote reviews of the PS of their love the lady will get tons of traffic and consequently she will work a lot and over time becomes mechanical...so their performance will get down while her price will get up and thus they will have less change to meet her and become their favorite 'buddy'''
There tons of 'callous dudes' in and out of TER;
3) Many simply do not care about writing reviews, they feel ashmamed to the idea that they are actually really paying Porn Stars to have sex with them, and usually the review is sort of 'mirror like Amlet's Shaskeapeare..."I'm really someone who pay for having sex"...not writing reviews allows them to perpetuated the illusion that after all they have nothing to do with paying for having sex.
4) People do this for pleasure and enjoyment: writing reviews is painful. Lots of dudes are just lazy person...and do not have time no will to deal with reviews and related stuff.

I think this its not just about TLC, but it apply to the high end market, to which the porn star are about the 70%.
BM