Full disclosure I work at VMware blah blah blah words are my own...
- vSphere is an eco system. There are TONs of solutions that it interacts with (and has really tight integration with). Running Horizon for VDI? Your only option is ESXi (although, Azure will be an option at some point here). There are tons of Ecosystem features that you REALLY can't get anywhere else, or are less mature on other platforms. Examples include
Microsegmentation with selective layer 7 service offload (NSX).
Platform residency like Pro-active HA (as well as vSphere HA and VMFS being far more resilient and tunable than other platform's options).
Platform features (vSAN isn't the only local storage replication system, but it's more tightly integrated and offers more data services and maturity than most systems out there).
Granular performance controls (DRS's algorithms are unmatched in the industry, NIOCv2, SIOCv2, highly mature APD/PDL detection at the storage PSP layer that would require a 3rd party software on most platforms). Using all this stuff I typically see 40% denser host utilization than people using purely free solutions (or just ESXi free).
Wholistic monitoring solutions and integrations (vROPS, Hyperic and LogInsight have crazy amounts of out of the box functionality across everything from hosts, to applications, to networking and external devices).
Hybrid cloud. Your replicating to a VMware partner in the vCloud Air Network like OVH or Softlayer (VCAN), or wanting to run Hybrid cloud operations to AWS (coming soon (TM).
Operational reasons. I can throw a rock and hit someone who knows how to manage ESXi and vSphere. There are a bazillion people are trained and know how to do not just basic Install configure manage, but also advanced troubleshooting.
Low cost 24/7 Enterprise support. I can support 3 hosts with 24/7 phone support for ~$1200 on an essentials plus bundle. Microsoft's lowest flat fee support option I've seen is 40K a year as part of an ELA.
Lost cost (Essentials Plus is ~6K. For similar functionality I'd need to buy SCCM VMM which costs more and lacks a 24/7 support option).
Better driver/firmware quality control. VMware VCG doesn't blindly certify anything for a 5 pack of heineken (Realtek was banned for a VERY long time). Do to what customers use the drivers for (Never consumer use cases) they get more attention and there is a higher expectation.
The vDS is incredibly advanced and mature. Beyond NIOCv2 shaping functionalities that are a check box away, Advanced LACP, single click to setup CDP/LLDP send/receive. VERY rich load balancing algorithms hashes (not just a basic IP hash). Other platforms historically require 3rd party NIC vendor dependent tools do things like this.
Security/Compliance. There are platform features that are painful to replicate in other places, or if doable require endless amounts of scripts or 3rd party projects that may or may not be stable. There is a full DISA STIG for classified use case. Due to ESXi's tiny size (few hundred MB for the kernel) it's patch surface is tiny. Compared to platforms that minimum install is 20+GB who require monthly patching to remain compliant.
Mature native backup API's (VADP/CBT), Native Write Splitting API's (VAIO allows near zero RPO) and all kinds of fun platform features that allow for a rich 3rd party ecosystem.
Rich API's, and SDK's that are stable and managed by grown up's (This isn't docker where everything breaks every 3 months).
VMFork plus Photon allows for linked clones VM's that have zero net Memory/CPU/Disk overhead and can be booted in 400ms. This can be controlled by Kuberentes or Docker endpoints and gives you the benefits of containers, with the isolation of VM's (Performance control, microseg, visibility) with vSphere integrated containers.
Free OpenStack deployment that's a simple wizard to setup (vSphere Integrated OpenStack).
Also, ESXi is free and offers a nice HTML5 UI that's quite easy to use.
Anecdotally, it's just easier to deploy and manage, and tends to do weird things a lot less (and when it does you've got support to reach out to). The new VCSA vSAN bootstrap system, and the configuration assist for bulk setup is awesome. With a simple wizard I can have my entire cluster deployed in under an hour.