Generally speaking, the way that new competitors enter a given market is with low-cost options that are often inferior to established players. Then, as those entrants expand upmarket by offering better and improved products, the existing/established players abandon parts of their downmarket products to the new entrants. This cycle repeats until there's nowhere left for the established players to go. At that point, these upstarts can often replace the existing players and become the dominant ones.
I'm not sure if the above was a deliberate strategic move on the part of IdenTrust or not, but in cross-signing the Let's Encrypt certificate, it effectively killed off the potential for new players in the low-cost TLS/SSL certificate market because there's no margin in $0. [Citation needed] Further, because the purpose of Let's Encrypt is to serve the base level of the market [1] with no apparent desire (as per the parent organization which is effectively a non-profit/public-benefit organization) to expand upmarket. This move would appear to solidify (whether intentional or not) the position of larger players who cater to larger customers while keeping any potential newer players from disrupting the space.
Aside: I love Let's Encrypt and have about a dozen or so certificates issued through them that I am in charge of. They're awesome and kudos to their team for what they've been able to accomplish. When they first offered certificates, the 90-day validity period felt very restrictive. Now it feels great because the certificates are automatically rotated every 60 days per various automation tools and painful certificate renewals are very much a thing of the past for me.