You can pick from any number of business, technical or marketing decisions - and there are many to choose from - that contributed to its failure, but fundamentally, nothing was going to save OS/2 from its original sin.
The system was very good, stable, snappy. The main issue was the lack of software. Microsoft made deals with all hardware vendors and whenever people bought a personal computer they would find Windows pre-installed. At that time most of the people did not know what an operating system was, they just thought of a PC as an appliance, and most believed that Windows was the only option, that PC=Windows. Plus, computer and software were quite expensive, so why buy another operating system when your new PC comes with Windows for free. There are articles over the internet that claim that Microsoft, to kill OS/2, made deals with PC makers (HP, Compaq etc.) giving them OEM Windows licenses for free as long as they did not offer OS/2 as an alternative purchase choice to the end users, which basically meant 99% of people did not even know OS/2 existed. You would then find OS/2 in airports, banks, public transport etc. (all big organizations who were IBM customers) but it was not known by regular users.
This said the main weakness of OS/2 was the SIQ [1], single-input-queue. The user interface had a single queue for dispatching UI events, and due to a poor design, a single badly written UI program that got stuck while processing a call from the event loop, would freeze the whole OS/2 user interface. This was improved in later releases but never completely solved, even though admittedly in the last years of OS/2 this type of issues became quite rare.
[1] https://www.os2world.com/wiki/index.php/Databook_for_OS/2_%E...
Especially GeoWorks ... makes you wonder why Microsoft won. If you ever tried installing and using OS/2 on a normal (for the time) PC, you don't wonder why it lost.