it's simply older. On the access side, OFDMA + SC-FDMA framing and higher modulation which LTE utilise would have required too much processing power at the time 3G was standardised circa 1999. Also it's an evolving science.
As chips in the site equipment and the user equipment both get more efficient, they can process frames faster; with LTE we can increase the complexity of the channelisation, modulation and encoding, and across multiple carriers simultaneously in realtime, to use the radio resource more efficiently (At the expense of processing power, which is cheaper now than in 1999). With UMTS, each handset has to transmit the full symbol rate with a different code, which is also quite power hungry on both sides. So there isn't really anything 3G has over 4G (that I can think of).
In fact some ultra rural networks originally deployed as 2G only, are now being upgraded to 2G+4G (skipping 3G). Early 4G handsets like the iPhone 5 and those nokia windows phones around 2012 (both with no VoLTE support) can still drop to 2G for calls/texts (CSFB), and you can imagine how many of these are in circulation in poorer parts of the world. 4G also supports 1.4MHz channels unlike 3G, which is really beneficial in limited backhaul, limited power, situations.