As far as I see IdenTrust does not issue EV certificates at all: https://www.identrustssl.com/buy.html
As far as I see they do not even issue wildcard certs - but currently that’s also not planned for Let’s Encrypt.
And the other certificates (a normal one-domain cert and SAN certs) are also offered by Let’s Encrypt.
And they are not really cheap BTW too there: The cheapest one costs $66.
The only thing they offer is a ‘Warranty coverage of up to $10 Million’, but I don’t really now what they have to cover there. Nearly every HTTPS issue is caused by bad server config so I doubt they extend warranty for this.
Besides this the only benefit for IdenTrust are a few backlinks and a bit advertising (I mean their logo and their name mentioned at several places of Let’s Encrypt), but I don’t think that outweighs all the disadvantages.
So to conclude this: I have no idea why IdenTrust does this.