In the mad rush to re-arm following the Tukayyid Cease-fire, the Great Houses focused primarily on Battlemechs, and on optimizing their existing Mech production lines in order to better match the Clan threat. However, this meant that their militaries were neglecting complementary units such as aerospace, infantry, and vehicles. Clearly, the modernization and optimization efforts needed to be spread more broadly, and 'Mech forces made up only a fraction (and in the case of planetary militias, a very minute fraction) of Inner Sphere forces.
Two competing factions quickly rose, with one camp emphasizing the need to upgrade the Inner Sphere's laughably outdated Aerospace units, in order to gain a qualitative advantage over Clan forces. It had not gone un-noticed that Inner Sphere pilots had a significant experience advantage over their Clan counterparts, in most cases completely negating the superior technologies fielded by Clan aerospace units.
However, the competing camp argued that pouring resources into Aerospace units would have very limited benefits for their extreme cost -- aerospace fighters being notoriously expensive to manufacture and maintain, and even the best fighters being quite fragile and short-lived in direct combat. Furthermore, aerospace units were just another small fraction of Great House regiments, in most cases an even smaller fraction than 'Mech units, and one that saw even less combat than 'Mech did, per unit.
Instead, the argument was to focus on the vast bulk of military forces used by every Great House and minor power. A series of crash programs have already gone into effect to enhance the capabilities of the Inner Sphere Standard Battle Armor, aka the "gorilla suit", but it was obvious that such programs would take years to come to fruition, and that few (probably none) of the Great Houses were interested in equipping more than a few elite infantry units with such battle armor. Instead, it should be far simpler to take the existing vehicles, transports, fast scouts, VTOLs and the like already in service, and supply them with 'field upgrade' packages that could vastly improve their combat effectiveness, and their ability to meaningfully support both infantry and 'Mech forces.
As an added benefit, the most cost-effective (and tonnage-saving) upgrade would be the use of fuel cell engines, a mature technology already widely in use in civilian applications. Although the Star League had chosen to use either fusion engines (for the weight savings and effectively infinite power and range), or Internal Combustion Engines (for cost savings, reliability, and substantially greater strategic range than fuel cells), Fuel Cell technology had continued to improve throughout the Succession Wars, and were not only highly reliable, but where they had once barely a third the strategic range of a comparably-rated ICE, modern Fuel Cells now had about two-thirds the strategic range, arguably making them superior to ICE power in combat applications.
What's more, the near ubiquity of modern Fuel Cells in civilian vehicles, having replaced ICE power in all but the most remote and periphery systems, meant that civilian vehicle manufacturers could be tasked with producing portions of the upgrades, and the less exacting specifications for vehicular armor meant that civilian heavy industries could be tasked with producing newer armor compounds. In effect, this meant that existing armaments firms -- many already stretched thin -- would have to focus only on expanding weapons production in order to produce these upgrades. From a practical perspective, this meant that Great Houses could upgrade six to eight times as many conventional vehicles with fuel cell engines and modern lostech weaponry, compared to upgrading their even their existing aerospace fleet with XL fusion engines and lostech armaments -- and positively impact a concomitantly larger portion of their industrial capacity, as opposed to just a handful of specialist aerospace manufacturing firms.