The reason is that we want to prevent that we accidentally recognise tags as such while it wasn’t meant to be, for example, in the following sentence:
Some things to do #priority for next week's meeting in london
Is the tag we should parse:
#priority, or should it be
#priority for next week's meeting, or perhaps
#priority for next week's meeting in london?
We decided to keep it simple and restrict tags to single words only at the moment. Having said that, we do plan in a future update to offer more options in this area. Stay tuned!