Because it's a weaker joint, vs. factory formed line, or a proper double-flare and union, and proper surface prep. is necessary (essential) to ensure a durable leak-free repair.
And many shop's won't install them, due to potential liability.
With ^^ that said, it's entirely doable to use a compression fitting on fuel line, if you have straight section of clean, original line that you can properly prepare -> and mate the compression fitting & new replacement line to.
Just make sure the compression fitting is rated at least 2-3x above max. pressure in the line, follow the recommendations in Stillrunning's thread he linked, it's an excellent read if you are going to DIY.