It's an expression, which in the nominative case would be "данные социологического опроса" (survey data). The dative case is required by the preposition "по" (according to). The expression "социологический опрос" stays in the genitive case because it defines the word "данные". "Социологический" is an adjective that defines the noun "опрос", that's why it takes the same form (genitive). Maybe you're confused because in Russian there is a noun 'данные', which means 'data', and an adjective 'данный', which is the synonym of 'this'. These are absolutely different words.

'Data' in Latin is plural of datum, 'given'. The same in Russian: данные - given[things].

Because it is dative of "данные" ('data'):-). По данным.... Accoring to the data... 'data' here means 'information collected'. ....данным социологического опроса.... ...data of a survey...