There are reasons behind this request. The idea of bio is basic but it plays a role in privacy & security and straightforward user experience. I’ll shed a light on this:
1. The limit currently allowed (70), pushes users towards opening public channels to share just a few more words about themselves. This is problematic:
These channels contain user's personal information! And if in any case user loses access to their account, (being away for more than 1yr, forgetting 2-step verification pass...) the information will forever be stranded on a channel on the platform out of any access.
Telegra.ph articles face the same problem. Thus user personal info (bio) is safest kept in a place that’d go away with the account. That is the bio section itself!
Each of these channels occupies one more username on the platform. Channels that are almost empty and often contain only 1 short message.
2. due to short limit of characters, ppl often use links as a way to circumvent this. Often even normal links won't fit so they use short URLs (bit.ly or short.it). Their usage is currently understandable & this leads to compromise of the viewer's security. Because users end up clicking on links without knowing where it'll take them to!
If bio length is increased, there'll be no justification to use short URLs and usage of such URL will hint user against a potentially suspicious link.