Maybe they couldn't get an economic load on the 787 - too few pax - but enough for a 738.
Sal has been operating year-round by TUI at once-weekly since 2015. Apart from summer 2018, when the aircraft was switched to the airline's Boeing 787-8, the route has been operated by TUI's Boeing 757-200 aircraft.
Loads were generally high, eg
Summer 2018
May - 2531 passengers, average load 253, up 50% on May 2017 when the route was 757-operated
June - 2248, 281, up 16%*
July - 2234, 279, up 16%*
August - 2829, 283, up 63%
September - accurate figures not available **
October - 2555, 255, up 50%
* an additional operating day in these months in 2017 than in the same months in 2018
** the CAA made a pig's ear of the BRS figures for September (see elsewhere in the BRS threads)
It can be seen that as far as loads are concerned the market generated far more passengers than could be accommodated on a Boeing 737-800, and the Boeng 787-8 saw its loads increase over those of the Boeing 757 broadly commensurate with the seating capacities of the two types.
TUI is disposing of its 757s over the next year or so. In summer 2019 it has been reported that BRS's two based TUI Boeing 757s will be replaced by two Boeing 737-800s giving a base complement of four 737-800s and a 787-8 part-based for five days each week.
So that will mean a considerable drop in seat availability to Sal in summer 2019. It seems that the 787-8 will now be switched to the Dalaman route on Wednesdays having been originally shown to be operating to Sal as it did in summer 2018.
I haven't checked the overall Dalaman operation for summer 2019. In summer 2018 there were ten weekly flights spread across easyJet, TUI and Thomas Cook against the once-weekly Sal route by TUI. TUI has either calculated that Dalaman will nevertheless be a better business prospect than Sal for the 787-8 or there is some operational reason which on first sight seems unclear. Perhaps the change is based on forward bookings across the two routes for next summer.
I know a couple of groups of people who travel to Sal from BRS at least annually and they all commented how much they enjoyed the 787-8 in summer 2018. I don't know if they have already booked for summer 2019. If they have and did so on the basis that the aircraft would be a 787 again, they won't be happy bunnies if they find they are now flying on a 737, even if it is a MAX.