Because she's a pretty, stylish, intelligent person who works very hard, mostly keeps her mouth shut, has been more devoted to the Spanish monarchy and not embarrassing it than Juan Carlos for the past several years
, has produced and raised two kind, respectful daughters who are universally adored...
...and just as a side note the king appears to love her and benefit from that.
She's just an easy target. But if there are some people who
still don't like her mother-in-law and others who have made Sofia the previously most popular member, Letizia still has plenty of room to rise in esteem.