It isn’t clear why it has been dropped. The Deputy Speaker said only the Sponsor of a bill can withdraw it. I’m guessing (because www.parliament.uk certainly isn’t helpful) that it means David MacLean has realised it was a bad idea and asked for it to be deferred (perhaps so he can withdraw it at a later date) or someone else made him realise it was a bad idea.
All told it is good news for now but I’ll set a reminder to check it out on May 17th.
UPDATE: Martin Rosenbaum author of the BBC’s Open Secrets blog thinks MacLean withdrew from today presumably because he feels he can get more support on May 18th.