What makes the band of an antenna broader than others?
Very broadly speaking, it's the Q of the antenna (if seen as a resonant element) that makes it more or less broadband. The higher the Q, the "more efficient" it transfers energy, but the lower it's bandwidth. But there are many factors that confound that simple model. For one, it does not account for any losses in the antenna (e.g. due to high epsilon loading to make it more compact) or how efficiently it can actually couple to air. E.g. the way how to build a wire antenna is to use an L-C tank to make the wire resonant at the desired frequency. You can shorten the wire and by adjusting the L-C tank you can still keep it resonant. But the shorter the wire is, the less coupling you get into air. Unfortunately, there is no rule of thumb which antenna types are better than others. You have to actually try the different designs (or simulate them) to know which fits your needs better. If you have done many such experiments/simulations, you get a kind of feeling what might work and what might not work. But in the end it's just a guess and you still have to try... and you get often interesting results you wouldn't have expected.