With Nod forces dispersing at the end of the Second Tiberium War, GDI forces had to re-adapt to countering guerrilla tactics and insurgents. This change led to a bigger blow to the GDI armored forces than any Nod operation could ever hope to achieve: the epitome of GDI power, presence and domination, the mighty Mammoth MKII, wasn't able to keep up with this new era. With billions of credits wasted on maintenance and ferrying MKIIs from battle to battle, the walker was soon deemed unbearable for further deployment.
Then, the Third Tiberium War changed everything. Facing Nod troops able to stand their ground in a direct fight and the Scrin warping in entire armies, GDI commanders were desperately trying to get a modern successor to their most successful heavy armor ever. Even though lots of resources went into the development of the AW-12 Mastodon battle walker, its "cost effective" construction made it completely useless at taking over the MKIIs former combat role.
Only when a group of Steel Talon veterans, trapped for months in an old MKII production site by a Scrin technology harvesting division, jury-rigged a blend of the Mastodon and the MKII and paved their way trough enemy lines was GDI high command able to overrule the treasury board and authorize a new heavy walker for mass production.
You make me proud of this nickname again!