First Chen was changed by Yang Luchan and then later changed by Yang Jainhou and then Yang Chengfu and the original forms were no longer practiced by them and Yang Chengfu had more students than anyone else in the Yang family. Second Chen split Laojia Yilu, after Yang Luchan into Laojia Yilu and Laojia Erlu, so the chances are what Yang Luchan learned was not the Laojia Yilu we do today. Then Chen Fake developed Xinjia Yilu and Xinjia Erlu and the Chen family has kept training and teaching them all. They did not abandon Laojia just because Xinjia was invented and they see the merit of Xinjia so they did not ignore it.
Chen also has more weapons forms as well the Da Dao, spear, staff, jain, dao, double dao, etc.
Also it depends on what flavor of Yang you train
Mine, form Tung Ying Chieh had long form, 2 fast forms, staff, short dao, long dao and jain.
But there are other Chen forms out there that the family does not teach like the 108 and then the first Laojia yilu I learned was called Old style Shandong province Chen and it is laojia yilu with but a bit longer. And one of the Chen family was in Shandong teaching at one point and it is suspected that his form is still there and this to is pre the split of Laojia yilu into yilu and erlu.
And then it could simply be a the two families look at things differently
As my Yang sifu has said many times, “Chen is too low” and as a Chen family member once said “Yang is too high” thi to could have something to do with it.