IRS pub 17 says that pensions and annuities should be entered on 1040-SR lines 5a and 5b. On my completed form, line 5a is blank but 5b is not. Why is line 5a blank?

Line 5a shows the amount that is not taxable.  It is only for reporting purposes.  Line 5b is where your taxable pension amount should be placed. 

If your pension amount is taxable, it will show on line 5b. 

If the taxable amount on 5b is the same as the gross distribution on 1099R box 1 then it doesn't show the amount on 5a.

See IRS 1040 Instructions page 26 middle column

https://www.irs.gov/pub/irs-pdf/i1040gi.pdf 

It says........

If your pension or annuity is fully taxable, enter the total pension or annuity payments (from Form(s) 1099-R, box 1) on line 5b; don’t make an entry on line 5a.