The problem here is the number of bands; GSM was only ever on 4 frequencies (850, 900, 1800, 1900); you had a high and a low band antenna, and a combo PA which - if it wasn't using the same PA silicon - integrated both high and low band PAs in one package. Then came WCDMA; you added more PAs (one per band, as you couldn't make a wideband one good enough back then), widened your antenna switch a bit, and tweaked the high band antenna to be a bit wider for 2100 and all was well for a bit. The other 3G bands, besides 1700 that T-Mo USA used, were all still addressable by your current antennas.

Then came LTE. Being that lots of LTE frequency allocations were little nuggets released from other services (TV amongst others), and the inability to recycle the 2G/3G bands meant that there are a LOT of totally new bands. They go lower and higher than the old low band/high band antennas, and then there are the ones in the middle. Given the complex modulation schemes, the PA requirements are tight and hence you end up with even more PAs (though thankfully now the 3G PAs are mostly multiband capable so you get a little bit of room back).

See: http://en.wikipedia.org/wiki/List_of_LTE_networks

The antenna guys had to get even more creative at this point; I'm pretty sure the later apple phones have dynamically tunable antennas (ie they alter the effective length to suit the band in use)... but there's only so much room on a phone so even the 5S doesn't support everything in one model.

The new iPads are a single worldwide model - which is pretty amazing and I think may even be a first for an LTE device.