why generally ?
The over-riding principle here would be that Mainstage works on the assumption that all Instrument plugins are loaded and ready to play at all times. This may not be the case if a plugin is expected to load a new presert for a particular Patch.
Generally because in Mainstage one cannot directly transmit MIDI Program Change messages to a VI plugin. Should the plugin offer some other way to change presets, I guess it may be possible. Most do not facilitate changing presets at all. Mainstage does not currently offer any other ways to change the preset in a plugin within 1 instance.
Some plugins (Kontakt, Omnisphere for example) can operate multi-timbrally, with different presets assigned to different MIDI channels. With such a setup, it is possible to make use of the MainStage "Paste as Alias" function, in combination with a "multi-timbral MIDI Input" for the channel strip such that you can load only 1 instance of the plugin and access more than 1 preset.
So, depending on the particular plugin, generally, you have to load in a new instance for each preset. Most of the time, this does not cause any issues, apart from (potentially) a RAM issue for those memory-hungry plugins.