Pet Sounds:why CD mono version is so noisy?

I just bought Beach Boys "Pet Sounds" Cd that comes with both mono and stereo versions on it.Right away i noticed that the mono version is accompanied by a fair amount of hiss while the stereo one is very quiet.I have EMI Cd of all early Pink Floyd singles (all in mono) and the sound quality is outstanding,beating the stereo ones by a wide margin.Why is it not the same with "Pet Sounds"?As i understand both Beach Boys and Pink Floyd originally came out with mono versions first so i can't understand the noise issue on BB mono tracks.Thanks for your inputs.
Many original analog tape recordings have a certain degree of hiss that is quite noticeable. Prior to the 1980's almost all recordings were taped on analog open reel and the original Dolby hiss reduction wasn't used until the late 1960's and early 1970's.

Many of the CD reissues from the mid-1960's and before have been digitally reprocessed to remove hiss. It may be that the mono version of Pet Sounds took a more purist approach and they chose not to digitally remove the hiss. I can't state that for a fact, but from what you say and from what I know from comparing many original LP recordings to their CD reissues this would seem likely in your case.
I have the mono vinyl version of "Pet Sounds" and there is tape hiss, but I wouldn't call it noisy. Production techniques from the 60s would make tape hiss hard to avoid if you were doing numerous overdubs and mix downs. They only had an 8 track recorder and no noise reduction.

I honestly don't understand what the quality of any Pink Floyd recording has to do with "Pet Sounds". They weren't recorded at the same studios, they used different engineers, different producers and different recording equipment. And that doesn't even address the differences in the remastering stage.
Onhwy61,i chose Pink floyd as a comparison where original mono mixes from the same time frame (66-67) outperform stereo ones in quality and dynamics.They used numerous overdubs too.
I have on pretty good authority that they haven't cut a mono mix of petsounds from the original tape ina long time. So it's either been a dat or a safety copy. That could have something to do with it.