First of all, as stated earlier, the Germans lost half of their air transports before the Battle of Britain.
Air War Over France 1939-40 by Robert Jacksson said:
The Junkers 52 units had lost a total of 167 aircraft to enemy action, and a further ninety-eight had been so badly damaged that they were irreparable. For example, the unit that had carried Sponeck's force to the Hague - K.Gr.z.b.V.9 - had lost thirty-nine of its fifty-five aircraft, while in the same sector K.Gr.z.b.V.12 lost forty machines and was disbanded on the spot. It would take the German transport units a long time to recover fully from the disaster.
That is 256 lost in the campaign in the west, to add to the 86 lost in Norway. A grand total of 487 transport planes were commited to the offensive in the west 1940, so a little more than half was lost.
The Germans at this time had two divisions capable of air landing - 7. Flieger-division and 22. Luftlande-division. Both were thoroughly roughed up in the Netherlands. The Dutch managed to load 1200-1350 captured German paratroopers (of both divisions) on to their own navy and the Royal Navy and ship them to Britain before the army surrendered - about 10% of the German force committed.
With only ~250 transport planes, the Germans cannot drop both divisions, and cannot transport much, if any, heavy equipment. Without artillery and AT guns, the paratroopers will be at the mercy of the British reserves (which included a full armoured division) until thei can be reinforced by a naval landing.
In short, the two German airborne divisions at the time are roughed up, have suffered heavy casualties and lack the air transport to land in one wave, and above all, to get supplies and heavy weapons across.
And this is assuming total air superiority - which was pretty much imposstible, at any time the British could withdraw their fighters out of German fighter range, but still be able to operate over any landing area if they got too hard pressed, they never did, for a reason.
If landed, they might fight a week, maybe even two, before having to surrender.