No really, making a trailer is easier than making a teaser for a lot of reasons, but it comes down to only one:
Trying to show enough without showing too much. (for a teaser)
First off, what is a teaser ?
Full term is "Teaser Trailer", and to put it shortly, it is a short trailer, to "Tease" the audience, hence it's name.
One thing to avoid while making a teaser (in my opinion), that a LOT of movie teasers do, is to NOT show the same shots as in your trailer.
A teaser should give an "idea" of the ambience, the mood and the style of the content, via what it shows, via the music or added texts.
A trailer is there to let the player know what the product is about, which is why it usually contains clips of dialogues announcing the main strings of the plot.
The problem with trailers and teasers is that they tend to show the BEST parts of the movies, I will always remember that 2012 trailer that contained EVERY single heavy special effects shot of the movie.
Showing good stuff without showing the best, trying not to spoil, and trying to keep it short while fitting the right music...so yeah, making a teaser is not easy, especially when it's the first and you really do care about the project...