Watch "The Measure of a Man"
Get back to us on why there aren't 1000s of Datas walking around.
There also does seem to be a technological hurdle in creating a stable "positronic" brain to operate a sapient android. Soong was only able to do it twice
with Lore and Data (B4 (Nemesis) was too dim to be considered "sapient" or as "alive" as Data.) Even Data's attempt to do it didn't work out. Creating a positronic brain behind a sentient/sapient android seems to be more than the sum of its parts requiring an absolute level of perfection and even if you pull it off it seems that even then
there's a chance it won't work.
We can infer that since Data is able to be transported that the perfection need only be on the quantum level but replicators in trek operate on the molecular level. Even if you had a "quantum replicator" (requiring loads of energy to produce something) there still just might be a "quality" that is needed to reproduce a Soong-Type Android that a replicator might not be able to reproduce.
Keep in mind how absolutely special
Data is in terms of all of the androids we've seen in Trek. We've seen plenty but rarely depicted as self-thinking, autonomous, beings. Simply machines that "act human." Which is different than being
human. That's what special about Data, he's not completely driven by facts and programming. He's able to freely think and even create
. That's what makes Data special. He's a lot more than "just a machine."
That's not likely to be something that can easily be replicated even if you know absolutely everything
there is to know. It could be a delicate balance of all sorts of things on levels of creation that even if one speck of a chip or connection isn't 100% perfect
the whole thing doesn't work. The quantum level construction of Data's entire being pretty much being like our DNA. If something in our DNA is even slightly
abnormal you can be born with any number of defects hastening your death, shortening your life-span or crippling some aspect of your life. If you get to be born at all.
There's so much complexity to Data that he can't "just be duplicated." He himself even supposed that if his memories were transfered to a computer memory and then back to his body there'd be a "quality" or "substance" to them that'd be lost that only his own positronic mind can sustain.
One could -hypothetically- figure out how every neuron in your head is laid out down to quantum levels and then duplicate it making a perfect copy of your brain. But any consciousness in that brain wouldn't be you.
If we did this to preserve your life by copying you into a machine-body there's be a loss of consciousness or some "spark" that makes it all that is you special. What results is a poor imitation and copy.
Life is a lot more complex than just copying what is done. It may work on some level when we get to single-celled organisms, or similar absolutely tiny lifeforms but the more complex the system the greater likelihood there is a chance at failure.
Data is about as complex as it can get.