Why is computer called υπολογιστής instead of κομπιούτερ in modern Greek?

Everyone else has said the ‘what’. As to the ‘why’:

Formal Greek is resistant to Latin-based loans, and routinely translates them into Greek morphemes whenever it can. The resistance was always lesser in informal Greek, and in the last decade or so, the floodgates have opened up for technical terminology in English: Hellenic coinages often exist, but practitioners of IT rarely know them or use them.

That’s the overall trend. In the case of ‘computer’, the native calque hypologistēs has indeed prevailed over the loan kompiuter, and indeed so has logismiko over softgouer. As other answers have noted, the loanword is antiquated. But as Yiannis Tsiolis’ answer says, Greeks all know the word kompiuter (since they all know English by now anyway); and slang or jocular words are formed based on it.