I have tried both Inertia and Splade.
With Splade you get the Laravel niceties out of the box - no need to hack the code or reinvent the wheel if something doesn't work ok - and tbh it's quite common.
I don't want to write my app "twice" - once in JavaScript and once in PHP as it is with Inertia. There is a lot of duplication and non-standard solutions to things that are super elegant and straight-forward in Splade.
I will give you a real world example.
I needed to use Laravel Localized Routes - with Splade it works out of the box as if you are using a normal Laravel.
But I haven't been able to make it work with Inertia. Perhaps it can be done but the "plumbing" and extra coding necessary is not worth the time if I can use something that works without any hacks out of the box as is the case of Laravel Localized Routes with Splade.
Unfortunately, localized routes are important for my projects because I am not from the US and in my country we speak multiple languages.
This thing alone is a reason for me.
The other is the authors. The Splade author is super enthusiastic and active about Splade. The Inertia author just passed the project to the Laravel team - so I think that says a lot about the enthusiasm.
Another huge thing is out of the box stuff I use daily. This is so UNDERRATED.
You can see that the author of Splade has done a ton of real-life projects based on the stuff he offers out of the box in Splade. There is no to very little things I have to do from scratch when using Splade. The most common things are there ready to be used and customized.
In those no-to-very-rare instances of me wanting a specific JS functionality I do create a Vue component but that's rare.
I wouldn't go back to Inertia after using Splade.