Every year, Gartner releases their annual Magic Quadrant for Analytics and Business Intelligence platforms. In the report, they evaluate the strengths & weaknesses of the main service providers in the marketplace as well as providing a graph plotting these providers based on their ability to execute and their completeness of vision.
For the past 12 years, Microsoft has been slowly moving higher, but since 2016, the movement has been significant, with them being one of the leaders for the past 3 years and now pulling ahead to take the lead in 2019.
Since it's release in 2014, Power BI has rapidly evolved and grown to become a formidable challenger in the analytics and BI sphere. What a lot of people don't realise is that Microsoft is offering an ecosystem, not just Power BI. Low cost of entry has contributed to the high adoption rate of a product that is only 4 years old. Anyone can use Power BI Desktop for free. This single user offering includes data cleaning and preparation, custom visualizations and the ability to publish to the Power BI service. The Power BI Pro plan includes data collaboration, data governance, building dashboards with a 360-degree real-time view and the ability to publish reports anywhere and includes a 60 day free trial. Microsoft have also bundled it with the Office 365 E5 version, as well as embedded it in Microsoft Azure, so there really are a huge number of options available with total implementation costs being significantly lower than the competition.
Looking forward, Microsoft is now putting a lot of focus onto "Pervasive AI using Power BI". Their advancements in AI are being integrated into their BI stack for generic use, for both citizen data scientists and business analysts. Power BI’s automated capabilities with AI-infused experiences such as natural language, quick insights, image recognition, text analytics, and key driver analysis are new and has leaving the competition behind. They are leveraging the innovation from Microsoft Research with heavy investments into these integrations and constant updates. Add to this the frequent updates (Microsoft has been releasing monthly updates for the last 12 months) in a wide spectrum of categories, including reporting, modeling, analytics, data connectivity, data preparation, Power BI service, Power BI mobile and Power BI embedded as well as the investment in a broad set of visionary capabilities and integrating them with Power BI (examples include enhancements to augmented analytics, new AutoML features available in Azure Machine Learning and new Azure cognitive services) it's clear that they are committed to keeping their lead!
In a nutshell, there are a number of reasons, aside from ease of use (which users gave top-third ratings across all aspects for) and cost that so many of the estimated one billion Excel users worldwide are adopting Power BI as their go-to BI and analytics tool.