why is the cleavage in frogs holoblastic and not meroblastic?
Because:


Frog eggs are not yolk-rich. They are mesolecithal, they have an intermediate amount of yolk.
Yolk is concentrated within vegetal pole of the egg. That is why yolk-packed macromeres divide much slower than micromeres on the vegetal pole, which barely contain any yolk.



My sources:


https://kjcscientific.wordpress.com/project-excalibur/
Gilbert's Developmental Biology (2009)