There are multiple plausible reasons:
- Greed: No seriously. Number 1's tend to sell better because of a number of factors. What Marvel fails to take into account is Law of Diminishing Marginal Returns.
- Easy Access to New Readers: Unless I'm mistaken, none of us on this forum started reading back when Marvel was Timely Comics. People want to read from the beginning, but several hundred issues is a panic attack waiting to happen. The problem of this marketing strategy is that you still need to know the universe
My biggest problem is that Marvel wants to have access to new readers, but the problem is that these books aren't marketed properly. The only time I see comics marketed is within other comics.