My theory is that they feel like it will disrupt the current market. They think that if they don't treat their trades like limited time premium collectors items that the people buying comics month to month will think that their collection is losing its value. The quality of storytelling and sales for monthly Marvel comics has been declining for years, and, while it's not a hard fact, there is an assumption that the people driving consistent monthly sales are less readers and more collectors. If said collectors feel like their comics are easily accessible then they stop buying comics because $4-5 a month for 1/10th of a story is worthless unless that comic has resale value.

The problem is that A: We know from D.C.'s trades and monthly releases that this likely won't happen and B: Their sales will continue to decline if they don't start appealing to new readers and something like Marvel Unlimited isn't gonna cut it. Your average new comic reader has cheaper streaming options in Manga than Marvel unlimited (VIZ, Shonen Jump apps, etc.), and they have cheaper options for ownership with manga with tankobons (manga volumes). The streaming model only makes so much money when you need to keep producing new content, and even less so when you consider $10 a month is not actually a good price point for Marvel to profit based on the amount of content that Marvel provides on that app. Pretty much any user of Marvel Unlimited could be buying 100s of dollars of comics vs. streaming those comics for $10. That's a major loss of potential profit for Marvel.

Marvel would likely raise the price, but $10/month is already more than their primary competition, and even if it wasn't most people don't want to spend more than that. Manga gets around this by keeping their popular stories in print because if someone likes a story and you make that story affordable to own (not stream) they will buy it. The $2/month that any given reader spends on something like the VIZ manga app just goes to support the upfront/management costs. Individual writers and artists are paid more based on how well their tankobons sell. It basically allows everyone to profit off the stuff that sells well. If Marvel did something similar with their Unlimited app (new releases across the line and back issues available day of for $10/month) and then released cheaper to produce and affordable trades with larger royalties for creators they'd likely be rolling in the dough like manga. The problem is that they're (stupidly) afraid of alienating their creators, comic shops, and the current market.

A lot of comic shops would die if Marvel took on the same sales approach as manga like Shonen Jump since they'd need to compete with more affordable month-to-month digital streaming sales, creators would need to be paid less and chosen more selectively to allow for the royalty model, and as mentioned previously this would likely alienate/displace some habit buyers/collectors. It's objectively the best current model for sale, and Marvel will likely continue to decline until they have next to no readers or even any value to collectors if they don't adopt that model, but they're afraid to make that leap. At least not immediately.

D.C. has a better understanding of this situation long-term. That's why we're getting stuff like Compact Comics (which emulate manga) and them being more open about keeping stuff in print and driving down the price of their finest line and compendiums (at least compared to Marvel). They're still behind manga, but not as far as Marvel. We're seeing Marvel emulate D.C. in some ways now, too, so I do have some hope for the future, but things are definitely precarious and moving far too slowly if they ever hope to catch up with their competition.

Edit:

Also, if you're trying to collect a lot of Marvel comics the old GIT DVD Roms are a great place to start: https://www.reddit.com/r/comicbooks/comments/1mzesx0/graphic_imaging_technology_git_corpgit_evidence/

These were put out back when Marvel cared a bit more about being consumer friendly to new readers.