With tablets and smartphones, hardware support becomes a big issue. While hardware support for Linux on desktop PCs is not as much of an issue these days, tablet and smartphone manufacturers have an "embedded" mentality where a lot of hardware requires drives that are not openly available, and if they are, it isn't legal for someone else to use them in their OS.
Modern smartphones and tablets have lots of different hardware included, from cameras to motion sensors, GPS devices, touch screens, bluetooth transceivers, light sensors, proximity sensors, phone codecs, and more.
There is also the issue of fragmentation: different devices have very different types of hardware, varying to a much greater degree than desktop/laptop PCs.
When someone releases an alternative operating system for a modern smartphone or tablet, they are faced with the challenge of tracking down drivers for each and every one of the hardware devices on that smartphone or tablet, or simply not supporting some devices, or reverse-engineering their own drivers, and so on. Or, extracting the drivers from the stock OS on the device (where it can be difficult to determine what is legal and what isn't).
So alternative OSes on smartphones and tablets usually only support certain device models.
Note that there are "community ports" of Ubuntu touch to other device models - these are not officially supported but try to get most/some hardware working on devices other than those officially supported Nexus devices. The main reason they are not officially supported by Ubuntu is the unknown legal status of the drivers that need to be "borrowed" in order to get hardware working on those devices.