Because the new, urethane bushing outer diameters are the same as the outer diameter of the shell in the old rubber one. You would be over-compressing the urethane trying to jam into the shell. And where would the urethane find the extra room?...In the center hole where the inner sleeve is supposed to go. Good luck trying to get that in.

Two words: Follow instructions

FWIW: About two years ago I paid $109 for a 12-ton shop press from Harbor Freight because I was planning on replacing every bushing on my 245. It was well worth the investment because I saved myself hundreds of dollars in labor. Just this spring, my fellow BB'er Kourt99 and I used it to replace all his bushings.

It has more than paid for itself because I'd have given a shop $109 for 1.5 hours of labor.

http://www.harborfreight.com/12-ton-shop-press-33497.html