Methane is about half-way between Kerosene and Hydrogen in terms of Isp but it's far closer to Kerosene in density, which is critically important for overall achievable rocket stage performance (weight ratios). Liquid methane has about half the density of Kerosene but liquid Hydrogen has about 1/12th the density. Moreover, liquid Hydrogen is a super cryogen whereas liquid methane has a higher boiling point than LOX so handling, materials, and insulation requirements for LCH4 are comparable to LOX.