To understand the differences, I recommend that you consult the documentation: https://learn.microsoft.com/en-us/analysis-services/comparing-tabular-and-multidimensional-solutions-ssas?view=asallproducts-allversions
Regarding the "why", I saw many professionals just guessing since tabular models are designed to leverage in-memory columnar storage (we talk about modern data processing with the rise of cloud products)
They offer faster query performance and they are easy when it comes to integration with other Azure services.
I leave you with some links :