For Lager wasn't supplied by the brewery owning the pub, but rather by third parties. This was very unusual and wouldn't have been tolerated for UK-style beers.
At the present moment, however, we propose to consider briefly one or two of the problems that will come before brewers should lager beer become in any way a popular beverage. Up to now the sale has been of such a limited description that bottled lager has been hawked about in much the same manner as aerated waters. The one or two lager beer brewers in the country — who cannot be said to altogether "flourish ” — have practically had a free hand in vending their wares. Their transport carts have called at each public-house on the route assigned, and have freely taken orders for and sold the required number of dozens of bottled lager. So long as the trade is limited and uncertain it has not been deemed worth while to make any particular arrangements in the matter, so that we have witnessed the curious anomaly of tied houses, bought in many eases at absurdly high prices, being supplied with a certain amount of bottled beer by alien firms. It is true that in some cases an arrangement has been made whereby a small percentage of the invoice price handed in by wholesale lager-beer vendors has been refunded to the firms owning houses where these sales are effected, but, as a matter of fact, this system has only been in operation in isolated cases as far as the great mass of country brewers is concerned, and the policy which directed this toleration may easily be defined.
The Brewers' Journal vol. 35 1899, March 15th 1899, page 126.
Because the trade in Lager was so small, brewers weren't really concerned about that niche being filled by specialist Lager brewers. And here was a simple reason why brewers wanted their pubs to have a supply of Lager.
Supposing half-a-dozen men stroll into a tavern for the purpose of refreshing themselves after their labours. Perhaps four or five will call for a pint or half-a-pint of beer apiece, whilst the odd man will demand a "lager.” If his beverage be not forthcoming, it is as likely as not that the whole batch would sulkily withdraw and leave the publican in the lurch.
The Brewers' Journal vol. 35 1899, March 15th 1899, page 126.
This is definitely what is likely to happen when one of a group of drinkers wants a specific type of beer that can't be supplied by the pub. Something similar happened in my university years, when I wouldn't want to drink in a pub which didn't have cask beers. While, ironically, Lager drinkers weren't so fussy as every pub sold the sort of fizzy piss they liked.
Not caring about buying in Lager from other brewers was likely to change when larger brewers started to produce Lager themselves.
As long as the consumption of lager is confined to such narrow limits there is no doubt that brewers could afford to extend a slight indulgence to the lager brewers, but when it becomes a question of pushing this class of beer on the part of large and powerful British firms, the whole aspect of the question is materially altered. The prices that have been paid for licensed property during the past few years preclude any idea of laxity in the matter of whose beer is to be sold within premises owned by brewery companies, and we only voice the general opinion of the trade when we say that no ousting of custom in English beers in tied houses will be tolerated by the owners of these houses. If lager beer take any hold on the public the profits of its manufacture will have to be divided between the lager brewers and the owners of the houses where it is sold, and it is just as well that this fact should at once be recognised so as to avoid future misunderstandings.
The Brewers' Journal vol. 35 1899, March 15th 1899, page 126.
And that's exactly what happened. Brewers either brewed Lager themselves or had agreements with Lager brewers. Though it would be well after WW II when Lager moved from a niche to the mainstream.