Because they missed a few years.

Because 1970 was Comicon #1.

If you look at their list on Wikipedia, you will not that they held more than one event in several years.

Two in 1970, two in 1975, thus the 2018 SDCC was their 51st held event.

My guess is that they're celebrating its incorporation and not when it was first held. Conventions are normally planned months in advance.

Time travel. The first was 70 so the tenth was 79. From zero to nine is ten.