1. There is a wavelength and penetration depth trade off. For better accuracy you need smaller wavelengths (i.e. higher frequencies) but at higher frequencies ultrasound pressure gets absorbed at a higher rate, making it challenging to get the required pressure levels deeper into tissue.

2. Ultrasound focus can deflect as it travels in tissue (tissue is a non-linear material and multiple tissue types impacts wave propagation). The deeper you try to focus, the more likely that there is a deflection; thus your focus point won’t be exactly where you were planning.