There are a few significant factors behind their dominance, methinks...
Luck/fortuna/kismet: probably the most important consideration of all as it's elusive and unquantifiable. Juventus were lucky in the sense that Marotta/Paratici stuck around as a combination for so long instead of one of them seeking newer pastures like Jean-Claude Blanc, that Buffon committed himself to the club, that Milan had a falling out with Pirlo (who was in many ways the on-field architect of Juventus' rise), that Barzagli worked out so well despite being a bargain bin signing, that Tévez fell into their lap and performed like a man possessed, that no other major clubs pursued Vidal with a lot of determination, that Pogba was out of a contract at United and so forth. A lot of planning went behind this, yes — it was by no means pure coincidence, but they were exceptionally lucky with regard to player acquisition and retention — whereas the decision-making at both Milan and Internazionale was horrid, and to make matters worse they just couldn't catch a break.
Consistency of ownership and “vision”: Juventus have benefited from major decision-makers pulling in the same direction ever since Andrea Agnelli assumed a more prominent role (Blanc, Marotta, Paratici, Nedvěd all learned to get along for the most part). The ownership and vision at Milan has been much more inconsistent in comparison — it was no longer the apple of Silvio's eye and Barbara Berlusconi's vision was to cut cost and asset strip (like selling Ibrahimović and Thiago to Paris Saint-Germain), then you had the Li Yonghong misadventure, and now Elliott Management. Constant change of direction/priorities, revolving door at the managerial/directorial position(s) after Allegri — hardly the recipe for success or consolidation. Internazionale has similar-ish issues, albeit not as profound, and they have been getting their act together in recent years (including the acquisition of Marotta and Conte, who were key protagonists of Juventus' ascent). A lot of ingredients are in place for them to test Juventus' dominance in the near future.
Organisational ethos, mentality, history of success and sensible decision-making: Juventus had won 25 Serie A titles from the inception of the league to the start of the current cycle (vs. 16 for Internazionale and 15 for Milan). Since the '70s to the start of the current cycle, Juventus have won almost twice as many Serie A titles as Internazionale and Milan combined: 14 vs. 17. They have almost always been a dominant force in the Italian domestic scene, made good decisions with two empires under Trapattoni and Lippi, and Milan and Internazionale's penchant for self-sabotage in recent years has totally cleared the field for them (apart from minor uprisings by the likes of Napoli or Roma that were easily brushed aside because of the sheer difference in capabilities between the institutions). If there's one club you'd trust to bounce back and enter a renewed period of success in Italy, it's Juventus because of their exceptional track record and footballing acumen.
Revenues: a bit like United in the early '90s, Juventus entered a period of sustained success at just the right time, capitalized on opportunities to expand all their revenue streams (collective rights is an issue that affects every Italian team but continued Champions League broadcasting and Allianz Stadium revenue really set them apart), and made the brand much more visible (which attracted a better class of sponsors/associates). Even after Calciopoli (when Internazionale was about to peak), they were either only slightly behind Internazionale/Milan or in a position to actually go toe-to-toe with the domestic competition. e.g. 2009 Deloitte Money League:
And once they started regrouping, they wasted no time in building a considerable lead (further fueled by Milan and Internazionale's diminishing status).
