Because the IP was liscnesed to a company to allow people to put it on their shirts as part of a hobby. It did not liscens anyone to sell shirts with those prints already applied on them, least of all a third party. That is, Disney liscensed a company to sell prints for sewing on shirts, not shirts. When you moved into the shirt market, you are competeing with Disney for a different lisensed product. Disney may offer a shirt with the same image of Mickey Mouse on it (or may wish to in the future) and you are competing and with copyrighted characters to boot.
This wouldn't happen if you were to walk into a Disney Store, buy a shirt, and later sell it on a second hand market as you have legally purchased a shirt with a print on it (as opposed to selling a shirtless print with a shirt on it for profit). Even if the shirt fetches a higher bidding price on ebay because it is a collector's item as Disney is in the market of selling brand new never worn Disney shirts and does not want to be in the market of second hand Disney shirts that some guy may have sweated in.
Furthermore, if you are posting the picture wearing the new shirt with the sewing print applied, but not with the intention to sell the shirt, but say, discuss the consumer experience of using Disney's product, then it also wouldn't count as here you are giving an opinion on the quality of Disney's merchandise, which may affect sales, but is not competitively selling a similar product. For example, if I have a blog that reviews movies and I say "Avengers: Infinity War was better than Avengers: Endgame", I'm not selling Endgame, but stating my subjective opinion on the film and that it wasn't the best foot forward compared to a similarly offered product. Product Reviews (in the United States) fall under a specific subset of protection against IP rights claims called Fair Use, which allows for a new product to be produced using copyrighted property so long as the product is material changed (for example, I'm if it was IP theft, I would have to sell the Endgame film in full. But since Disney is selling a movie and I am selling a review of the movie, we are not competing in the same market space).
Fair Use is considered an affirmitive defense, which means that the defense must meat a burden of proof in it's claim. Affirmative Defenses essentially are Defenses where you in part agree with the claim, but the dispute is on the context in which the action took place. Without showing evidence that the circumstances of the action are not such that make it unlawful, such defense would basically be pleading guilty. Suppose then you are accused of taking a cookie for the cookie jar. Ordinarily, the person making the claim would have to show it's not you and could never be someone else, thus your defense would be that maybe Jane stole them. Or the evidence hinges on proving "WHO unlawfully took the Cookies From the Cookie Jar?" In an Affirmative Defense, you would answer the charge by questioning by saying that both you and the complaint against you agree to a statement "You TOOK the cookies from the cookie jar" but have disagreements on his statement that "You UNLAWFULLY took the cookies from the cookie jar." because your mother gave you permission to have a cookie and have evidence to support that.
In many copyright cases that could be Fair Use, there is nothing stopping Disney from making the claim, so Disney can still file a suit and hope that their army of lawyers scares people into stopping even if you could have a valid fair use, as that is a burden you must prove. Of the three possible outcomes for the suit, either the matter is settled quietly outside of the courts and does not make case law, Or the courts find in favor of Disney OR Find against Disney. Disney wins in two of these outcomes, including the settlement of complying with a cease and desist notice (which is not a law suit, but a threat of a law suit should the practice continue) as you are doing what they want and a settlement will still show Disney will sue to other potential violators where as a losing case will create binding law (in common law systems, like the U.S.) that could limit Disney, but even then the victory is Phyric at best as Disney can usually afford legal fees incurred where as you are less likely to be able to afford to see a case all the way through to a favorable outcome. Thus Disney will sue in cases it would likely loses as they gamble that the threat of the financial impact of reaching a decision in a court is something that will scare you into stopping. This is especially true on some websites like Youtube which would be threatened with suits for hosting a third party video and until very recently (2016 or 17) Youtube would give into the copyright holders despite a valid Fair Use claim existing. These practices can be defeated if one can argue in a court that Disney is doing this unfavorable fair use (called and anti-SLAPP) but this relies on the defense having knowledge of the law and the time to press the matter... ceasing is much easier and cost effective.