There are a very limited number of high draft picks available each year - much less than the number of positions there are on a roster. It appears that the Patriots have chosen to devote those early draft picks on other positions, and utilize free agency and trades to fill their roster at WR.
When the Pats do use draft capital on a wide receiver it is more likely to be trading a draft pick for a veteran wide receiver.
For example, 2018 rankings of Pats top receiving yards:
1.) 7th round pick - slot - Edelman
2.) 4th round pick - RB - James White
3.) trade (5th round pick) - WR - Josh Gordon
4.) 2nd round pick - TE - Gronk
5.) free agent ($4mil/yr) - WR - Chris Hogan
6.) trade (player) - WR - Phillip Dorsett
7.) trade (5th round pick) - WR - Cordarrelle Patterson
2017 rankings:
1.) 2nd round pick - TE - Gronk
2.) trade (1st round pick) - WR - Brandin Cooks
3.) free agent ($4mil/yr) - slot - Danny Amendola
4.) free agent ($4mil/yr) - WR - Chris Hogan
5.) 4th round pick - RB - James White
6.) free agent ($3mil/yr) - RB - Rex Burkhead
2016 rankings:
1.) 7th round pick - slot - Edelman
2.) trade (4th round pick) - TE - Marty Bennett
3.) free agent ($4mil/yr) - WR - Chris Hogan
4.) 4th round pick - RB - James White
5.) 2nd round pick - TE - Gronk
6.) 4th round pick - WR - Malcolm Mitchell
2015 rankings:
1.) 2nd round pick - TE - Gronk
2.) 7th round pick - slot - Edelman
3.) free agent ($4mil/yr) - WR - Danny Amendola
4.) free agent ($3mil/yr) - WR - Brandon LaFell
5.) 4th round pick - RB - James White
6.) free agent ($1.1mil/yr) - RB - Dion Lewis
2014 rankings:
1.) 2nd round pick - TE - Gronk
2.) 7th round pick - slot - Edelman
3.) free agent ($3mil/yr) - WR - Brandon LaFell
4.) 2nd round pick - RB - Shane Vereen
5.) trade (player) - TE - Tim Wright
6.) ) free agent ($4mil/yr) - WR - Danny Amendola
Prior to drafting Harry, the last time the Pats drafted a WR was in 2016 with Malcolm Mitchell - in the 4th round, 112th overall. He was on his way to being a productive player before injury prematurely ended his pro football career, so I'm hesitant to call him a bad pick; more of a case of bad luck rather than bad scouting. Harry and Mitchell are the only wide receivers the Pats have drafted in rounds 1-6 over the course of the last seven drafts.
The Patriots simply appear to prefer to build their roster at the wide receiver position through other available means. There is a finite number of draft picks; a team cannot fill a roster strictly via the draft and undrafted rookie free agents.