You're overwriting the file every single time you write a line. That's the difference between opening the file with 'w' permissions and 'a' (write-append) permissions.
There's no need to open and close the file every single time anyway. Just move that outside the loop.
cr = [1:10; 11:20].'
counts = 101:110;
fileID = fopen('conc.dat','w');
for k = 1:length(cr)
row = cr(k,:);
conc = counts(row(:,1));
fprintf(fileID,'\n');
fprintf(fileID,'%12.f\n',conc);
end
fclose(fileID);
