This is where I get annoyed with identity politics. If there isn't a qualified Hispanic candidate for president, then Hillary has no business picking one. It's as simple as that. It's not her fault that the Democratic Party has been pretty bad with getting Hispanic leaders on the national radar. It's not her fault that Loretta Sanchez doesn't have the same talent for national politics like Kamala Harris does. It's also not her fault that the Castros live in Texas, and that Antonio Villaraigosa is essentially a pandering crook.
Hillary is doing her damned best right now to make herself as viable to the Hispanic community as possible with her positions on immigration. Obama is pretty much going as far as he can go in elevating talent like Castro and Tom Perez to positions of power within the federal government. What more could they possibly do?
Are there even any Democrats on the level of a Sandoval or Martinez?
One factor is that Democrats are more reliant on winning in majority-minority areas.
That leaves them with many members of Congress unable to appeal to enough people to win statewide office. It's the reason there are as many African-American Republicans in the Senate as African-American Democrats.
There is also an element of luck. Menendez has been in statewide office longer than Rubio, Sandoval, Martinez or Cruz, and was the highest ranked Democrat in the foreign relations committee. But he's also under investigation.
