Why eating octopus is banned in many countries?

Octopus eating is banned in only Islamic and Gulf countries. As consuming octopus meat is forbidden according to Islamic tenets. 

As per the Shiite religious sources conferring to consumption of halal and haram meat, a handful of general rulings and standards outline the norms for eating the meat of land animals, birds, sea creatures and so on and so forth. 

The ruling condition for consuming sea creatures, the ruling precondition that categorizes sea creatures as halal and thus fit for consumption is the prevalence of scales on their body. This rules out octopus and makes it haram, the general ruling of impermissibility. 

As per the Wasa’il al-Shia, Vol no. 16 and page no. 397 “Given a fish we are, that do not bear an extra layer on them”. To which the Imam replied, “Eat those fish that have scales and do not eat the ones that don't have an extra layer on them”. 

A point to be noted here is the legal difference between consuming halal and conducting business dealings in it. Haram decrees octopus as inedible but it doesn't prohibit the fishing, selling, and purchasing for the sake of philosophies and market reasoning. 

Its a pretty common occurring, and similar occurrences include for eg blood.