2 reasons
1) it takes the fun out of looking, searching and finding that rare jersey, not really counting the pre 1990s jersey like the Birds, Worthys, Wilt, etc because you absolutely cannot find those jerseys, but its more for the mid 1990s jerseys.
take for example the 95 and 96 All Star jerseys, I saw a dude with a Terrell Brandon authentic 1996 Fiesta all star Champion and damn near had an heart attack, now M&N can reproduce them at will and they are not so rare anymore. Another example is the 98 Alternate Bucks, seeing the original Champion one damn near brought me to tears.
I dont mind when they bring them back in game as hardwood classics and you can buy the current jersey with the current players, I think thats fair game, but when you now own a Glenn Robinson 98 alternate Bucks as a M&N, it diminishes the person who worked hard to find the originals
2) the second is the utter low qualityness of these reproduction of these jerseys. Take a look at the Vancouver Grizzlies they decided to put out, its just disrespectful to the art of making jerseys and you can clearly see that they just want to make money and not preserving quality
the only good reason ive heard was from my boy who was like 6'6 and 250 lbs, there was no way you could have get any of these 1990 Champion jerseys for his size, so M&N reproduction was his only way to go since they can now make them in 3XL
other than that, you will not catch me 5 ft of a Mitchell and Ness a jersey. IF you ever see me at a M&N store slap me and spit on me, I have failed as a man