The M270 is less mobile than the HIMARS - the ‘HIM’ in HIMARS stands for ‘High Mobility’.
The M270 entered service in 1983, while the M142 HIMARS entered service in 2010. They both use the same ammunition, but the HIMARS carries/fires only half as much as the M270.
From what I can tell, the other main difference is that the M270 is a tracked vehicle, while the HIMARS is wheeled. The wheeled vehicle will be faster on the roads - 64 km/h (39.8 mph) for M270 vs 85 km/h (53 mph) for HIMARS - while the tracked vehicle will be more capable off-road. So the HIMARS is likely better for “shoot and scoot” operations.
Additionally, tracked vehicles are transported by train or mounted on a wheeled trailer for longer distances to avoid wear and tear. The wheeled HIMARS can probably drive itself to different parts of the country.