Telus has decided for a variety of reasons, likely related to quality of service, that they sell their 4K Optik TV, which has multiple video streams, only where 50 Mbps is available.
This is the correct answer. 50mbps is the requirement for 4k to support multicast Optik TV so a customer can watch a 4k stream or even multiple 4k streams while still having enough bandwidth for your normal internet connection. For example: a customer who can only get up to 15 or 25 mbps as a max attainable speed on their line could tune in to a 4k stream on Optik but their internet bandwidth allocated for streaming/browsing/downloading would slow down significantly. The minimum 50mbps plan requirement is to ensure the reliability of both products (internet and TV).