Few reasons:
- Blizzard team responsible for Diablo are trying everything they can to regain fanbase goodwill.
- Diablo 4 will be combination of D1 inspired atmosphere, D2 inspired mechanics and D3 combat feel.
- "Remake" doesn't always mean complete remake. They could just remake graphics like in Demon's Souls and leave mechanics untouched.
- Remaking D2 could be good training before release of D4
- Remake of graphics with old mechanics should be fairly quick much quicker than making D4.
- D4 is YEARS away from release