If the Stranger Things license was lost, can't the developers at least bring back the cosmetics?

Without seeing the exact contract, no, but I can take a guess.

Before doing so, however, I'd like to clarify I am not a trained, practicing lawyer, and while I have an A-Level in Law and am working on my Law Degree, this is all not legal advice.

Netflix could argue in a court of law that the Stranger Things cosmetics returning to the store would intice players into purchasing cosmetics, even if just with Shards, and would put them into a spending mood and thus would be a marketing tactic for BHVR to increase player count and sales. Moreover, they'd be doing this all with property that isn't their own. There's a reason why every licenced trailer, teaser, tome, cosmetic collection, etc. always has the copyright of the licence holder.

Without licence (Alternative word, permission) to use their content, Netflix could sue for Copyright infringement, and using the argument earlier would also be able to claim damages equal in value to whatever their legal team estimates the boost in revenue gained by having those cosmetics in the store is.

Moreover, every licenced cosmetic and chapter purchased requires a percentage of the revenue to be sent to the copyright holder as a "royalty" - an amount of money paid to the original copyright holders as payment in exchange for the allowing of their Intellectual Property (IP). This is outlined in the contract both parties formed and signed to enable the use of Stranger Things within Dead By Daylight. Selling these cosmetics for shards would still require BHVR send the royalty, as per the contract, and so they would be losing money just for keeping them in the store.

Finally, another reason could be that they legally cannot. It doesn't matter if the cosmetics are for sale for Iridescent Shards or Auric Cells, under the games End User Licencing Agreement (EULA), both are defined under "Digital Currencies" which are used to purchase "Digital Goods". I'd suspect something similar would be in the contract. While the contract is valid, BHVR are able to sell the cosmetics (the "Digital Goods") however once the contract has expired (or has been broken) they are no longer able to sell these cosmetics for any value whatsoever.

Again, I am not a lawyer, and despite me having studied the law for the past few years now, I have not been trained as either solicitor or barrister and this is not, and should not be interpreted as, legal advice.

Edit: I felt like doing some research, so if BHVR were to sell the cosmetics again and be found in breach of copyright infringement, here's what would happen.

Before that, however, we need to define the different types of crime. These are outlined in the Canada Criminal Code.

A summary offence is the least serious, tried in smaller courts for smaller values. The general rule is that the value of this crime cannot be more than CA$5000, and as such the punishment cannot equal more monetary value than CA$5000, a prison sentence of more than 2 years, or both, unless otherwise stated by the law.

An indictable offence is the most serious, tried in larger courts for larger values. In the UK, this is usually tried in the High Court (Criminal Division), although could be tried as a civil case under the Multi-Track system. Essentially, it's anything over CA$5,000 (again, unless stated by law), and so the punishment cannot be anything less than CA$5,000, 2 years imprisonment, or both.

The punishment for copyright infringement in Canada is outlined in s.2(1) of the Copyright Act (R.S.C 1985). There are two different punishments, which relate to the severity of the crime.

For a summary offence, the punishment would be a fine of up to CA$25,000, 6 months imprisonment, or both (s.42(2.1)(b)).

For an indictable offence, the punishment can be a fine of up to CA$1,000,000, 5 years of imprisonment, or both (s.42(2.1)(a)).

So yeah, facing 5 years of imprisonment, a 1 million dollar fine, on top of being able to also be sued for damages and a potential breach of contract? I wouldn't risk it if I were BHVR.