While "faster" is certainly a part of it, that is not the big deal, AFAICT.
The major driver is that there are large simplifications of the communications infrastructure that make both base stations and the controlling infrastructure easier and cheaper per bit transported. Systems engineering in the different generations is different.
Timing, for instance: The Old Way was to have the SDH-derived PDH clock in a E1 or T1 circuit steer the reference clock in the RBS. Which was fine when every telco had a SDH net (because this TCP/IP thing is not something we believe to be usable for Real Telecoms) but since most telcos today provision E1's over IP it is something of a nuisance having the only part of your actual telephone infrastructure not being decommissioned rely on something that's now expensive to provision, with questionable timing accuracy to boot.
Therefore most new build systems use PTP over Ethernet for clock steering. Now, you can put a simple IP router like the Cisco ASR920 in your base station rack, pull 2 dark fibres with 10GE to it from your backbone, and instantly, you have connectivity, timing, and clock backup (it's got a GNSS receiver as well). The ASR920 is not cheap to you or me (USD 5000 list, plus software licenses), but compared to what the telecoms manufacturers made the telcos pay for SDH, it's basically for free..