I've always wondered if scolipede with its hidden ability speed boost is allowed in pvp, then why is blaziken with its hidden ability speed boost is not allowed?

Because Blaziken is practically broken with that ability and hits like a truck. I guess pvp experts can answer you better.

Speed Boost, after 2 turns, its impossible to Revenge Kill and requires priority to KO it.

 

The ability is therefore broken, its almost impossible to stop.

 

Hope this answers your question!

 

Ozai

PRO is not showdown, but many rules and bans are the same. You can look if Blaziken is banned in Showdown as well and look for the reasons there. Like said, PRO and Showdown is not the same - but something close and the reasons and arguments are very similar normally.

 

As addition to already mentioned stuff for example:

https://www.smogon.com/forums/threads/xy-ou-ban-of-blaziken-and-deoxys-n.3492497/

Speed Boost Scolipede was primarily used as a Baton Passer as it provides greater defensive utility than the other options (Ninjask, etc.) - for example, it's able to soft-check Gengar and Serperior. Offensive Swords Dance Scolipede sets suffer from a lack of overall power - they are unable to break through common Pokemon like Ferrothorn and Skarmory at all. Blaziken has no such issues with its brilliant offensive stats, and becomes a powerhouse with much less investment required - a single free turn and Blaziken is +2/+1 and threatening to immediately end the game. While it can still be stopped by a select few priority attacks (ex. Azumarill Aqua Jet), it is nigh-impossible to handle defensively and was accordingly banned from Ranked PvP.