Two things.
One is the weight of stablizer. Since there are literally hundreds to choose from, it can be difficult to choose the appropriate one for your project. This can greatly affect the stiffness of the design. Some people find one they like and use it on everything. Not a good idea.
The second one is the design itself. All designs are not appropriate for all situations! You need to choose a design with less stiches for something you want to remain flexible, like a t shirt or quilt block. Stitch intensive designs are great for tote bags, hats etc. If a design is very stitch heavy, and your software or machine has the capability, you can enlarge the design about 5-10%. Be careful! There often are two options. One option will make the design larger AND add additional stitches to compensate for the larger design. The other option (which is the one you want) will make the design larger but keep the SAME number of stitches as the orignal design, thus spreading them out a bit and making the overall design less dense. If you choose to do this, it is highly recommended to do a test stichout before putting it on your actual project.