It seems to me that the image is quite adequately sharp given the lighting conditions. Some things affecting the sharpness of a photo in low light:

* Almost any lens is going to be somewhat soft at its maximum aperture. As far as I know, the Canon 50mm/1.4 at f/1.4 is a bit on the soft side relative to other fast primes (even compared to the cheapo 50mm/1.8!)

* The larger the aperture, the tighter the depth of field - most of the things in your photo are simply not in focus due to the razor-thin DoF at f/1.4.

* The less light, the less contrast and the less accurate the autofocus mechanism is. And the larger the aperture, the easier it is for even a tiny focusing error to throw the focal plane slightly off.

* Related to the previous two bulletpoints, moving the camera even slightly (eg. when doing "focus-and-recompose") might well be enough to cause noticeable misfocus. Using the non-center autofocus points might help, but they are often less accurate than the center one.

* The higher the ISO, the softer the image due to noise reduction. This is something that can be tweaked in post-processing if you shoot RAW, but it's always going to be a balancing act between noise and softness.