Cause you can use "gift of the martyr" on OZ to pass its attack on piranha giving it 5000 attack. Where you can't achieve the same attack w blue-eyes in a 2card combo
They have no plans to take out the card box anytime soon. It will likely be available throughout the Paradox Brothers event. There is no reason to use Master of Oz over BEUD other than MAYBE getting the fusion summon points, but if you're using Serket as recommended, it doesn't matter.
Master of Oz only requires 2 materials, while BEUD requires 3. Also, Mater of Oz gives Piranha Army exactly 10,000 damage, which is enough to get the Over 9,999 damage bonus. Basically, if you are using Summoner of Illusions, there is no difference, but if you are fusing, Master of Oz requires 1 less material for the exact same outcome.
Understand your point but please see writer mentioned "Can be alternatively special summoned on your final turn by using Mystical Beast of Serket and Temple of the Kings."
if you plan on summoning the high attack fusion monsters through serket or summoner of illusions, then they're both pretty much the same, but Oz is better imo since it only needs 2 monsters and fusion summoning gives bonus pts instead of just special summon through serket/SoI.