So just why is LTspice better than other SPICE programs?
Economics. If I write a SPICE program for an EDA company, it's possible to gross some smallish number of millions of dollars revenue. But if I write the same program for an IC company and it's used to both design and sell the ICs, then that same simulator is critical path to 1.5 billion dollars in revenue.
Sure, but what's the big TECHNICAL difference between LTspice and any other SPICE out there? Aren't all SPICE's based on Berkeley SPICE and essentially the same solver?
LTspice is a dramatically improved version of SPICE over any other SPICE program available. While most of these improvements are proprietary, here is an article that discusses a few of the differences between LTspice and other SPICE programs:
SPICE Differentiation, LT Journal of Analog Innovation, 2015
Note that article uses screen dumps from an old version of PSpice for comparison to the better solver in LTspice, but the netlists are machine readable and you can run them in the current versions of commercial simulators to see that (i) the problems persist and (ii) SPICE solver development as stopped at SPICE software companies.
OK, I get it. But what about that GUI?
LTspice's GUI was based on a statistical analysis of the keyboard entry and mouse motion required to enter a schematic. Besides the overwhelming empirical success of LTspice's GUI, it actually is the easiest GUI to use for schematic entry.