The trend in high end mountain bikes is to have 1 front ring and eleven rear so only 11 in total. This is less complex and less annoying. The problem is that

The trend in high end mountain bikes is to have 1 front ring and eleven rear so only 11 in total. This is less complex and less annoying. The problem is that you loose out on some high or low gear ratios that you might want. There are limits to the range of gears that are possible in that setup. So that is why front derailleur's were used. It expands the range rather than just the count of gears. A 30 gear bike has three rings on the front. The outer and inner are only supposed to be used on the outer few, or inner few rear rings. Otherwise the chain will be stressed and possibly break. Only the middle ring is used with all rear gears. So you actually have only about 16 usable gears. A more modern design is to use two front chain rings. This does not strain the chain so much, and you can use the full range of rear gears. It is also much more natural feeling to shift the front gear. A 20 speed bike actually has more usable gears than a 30 but probably less range. A further iteration is to have a larger inner ring on the rear derailleur that is a bigger step up from the last. Eleven speed cassette's try to give enough range on the rear derailleur to only need one front ring.