To answer honestly, it's because the first sentence only uses common and easy to spell words.

Sphinx of black quartz, judge my vow. (29 letters)

Waltz, bad nymph, for quick jigs vex. (28 letters)

A perfect pangram contains every letter of the alphabet only once and can be considered an anagram of the alphabet. The only perfect pangrams of the English alphabet that are known use abbreviations or other non-dictionary words, such as "Mr Jock, TV quiz PhD, bags few lynx", or use words so obscure that the phrase is hard to understand, such as "Cwm fjord-bank glyphs vext quiz", in which cwm is a loan word from the Welsh language meaning an amphitheatre-like glaciated depression, vext is an uncommon way to spell vexed, and quiz is used in an archaic sense to mean a puzzling or eccentric person. It means that symbols in the bowl-like depression on the edge of a long steep sea inlet confused an eccentric person.

https://en.wikipedia.org/wiki/Pangram?useskin=vector

As I've said before. Quick brown fox is chill and comforting, like a pleasant autumn day at the park or perhaps a forest. Sphinx of black quartz is objectively a million times nerd shit and uncool. It tries too hard to be cool. Like a mom or dad trying to use the new slang that the kids have been throwing around. And because they don't know how to use it, the slang is made to be out of place, uncool. Maybe if it was in the midst of some great, terrible, perilous story of bravery and heroics - a choice line said when it was most needed - it would be amongst good company. But it's not. It's just a sentence used to display the letters of the alphabet. So diluted and stale. Through this constant repetition, this constant exposure, it has lost potency. Venom in blood so carefuly exposed a hundred fold as to experience no symptom. But Quick Brown fox suffers no ill side effect, because it was bred for this purpose. It knows what it has been made to do and does it with pleasure.

Thanks for reading.