A low SWR bandwidth is a consequence of shortening the antenna for mobile use. SWR bandwidth is related to the antenna system's Q factor, the ratio of energy stored to energy dissipated per cycle. A higher Q means less SWR bandwidth.
Decreasing Q will increase SWR bandwidth. That can be done by decreasing the energy stored, or increasing the energy dissipated.
A smaller loading coil will decrease energy stored, but will require a longer whip.
Distributing more current over a longer length of the antenna will increase the energy dissipated by radiation. This can be done by moving the loading coil up, thus leaving a longer section of antenna below the loading coil with high current. Or a capacitive hat at the top of the antenna increases current overall. But in either case there's some cost to be paid: higher wind loading, a longer whip, or a bigger loading coil with associated loss.
Of course energy dissipated can also be increased by increasing losses, but that's not really desirable. So if it makes you feel better, a low SWR bandwidth suggests a good, efficient antenna!