The problem is that T-Mobile is already running very lean. According to Roger Entner, an analyst at Recon Analytics in Boston, T-Mobile's costs are "cut to the bone." Sprint, with a large overlap of towers and employees in the U.S., would have an easier time eliminating duplicate equipment and employees. With a higher amount of synergies available, Sprint has been rumored to have already agreed to buy T-Mobile in a deal that values the company at $32 billion.
Iliad might make more money per employee than T-Mobile, and might also have a higher profit margin. But T-Mobile has a high-speed LTE network that is the fastest in the states. Iliad uses roaming agreements with other carriers to fill out parts of its network. Certainly Iliad couldn't have really thought that Deutsche Telekom, which owns 67% of T-Mobile's shares, would have agreed to the deal.
Iliad now has two choices. It can be serious and raise its bid to a price that Deutsche Telekom's board would have to seriously consider, or it could slink away and let Sprint work on finalizing the offer it is expected to announce in September.