As prospective oncology drugs progress from Phase I to Phase III clinical trials, the difference between projected and actual patient enrollment durations typically increases. In most cases, oncology clinical trial delays are a direct result of patient recruitment challenges.  This is particularly true when multiple companies run competing clinical trials. Adam Bianchi, Ryan McGuire & Rochelle Gagg at Cutting Edge Info, analyses the two main factors impacting duration are the level of difficulty of trial-specific patient recruitment and the number of patient visits required by a trial.

https://international-pharma.com/wp-content/uploads/2013/06/2-Patient-Recruitment-Driving-Lenght-and-Cost-of-Oncology-Clinical-Trials.pdf