The Type 2 PTO (1-3/8"x21 spline 1000rpm) was introduced to transmit more power as implements started using 75hp+ and for the longest time Type 1 (1-3/8" 6 spline 540rpm) was considered 80hp max.  Currently ISO500 allows type 1 up to 100kW or approx 150hp.  The latest standard has max thrust values for shaft telescoping based on shaft type and hp.  A lot of 540rpm snowblowers were run on 160+ pto hp tractors, it just takes a big driveline.