In your first case, you've given it one value for rho and one for theta. [I'm assuming the "theater" on the third line is a typo.] Since you only give the coordinates of one point to polarplot, it only plots one point.
As written your second case creates length(rho) individual lines, each of which represents one data point. The first is theta = 120, rho = 0. You can confirm this by adding a marker.
polarplot(theta, rho, 'o');
If you want to connect all those points with a line, repmat the scalar theta so it is a vector the same size as rho. That will create one line containing all the data.
thetaV = repmat(theta, size(rho));
polarplot(thetaV, rho);
One more thing to note is from the polarplot documentation. The theta input should contain "Angle values, specified as a vector or matrix. Specify the values in radians. To convert data from degrees to radians, use deg2rad." You look like you're specifying theta using degrees not radians.
